Patent Publication Number: US-7721596-B2

Title: Apparatus and method to monitor slurries for waste re-injection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a Divisional Application of U.S. application Ser. No. 11/409,831, filed on Apr. 24, 2006, which claims priority to U.S. Provisional Patent Application Ser. No. 60/703,672, filed on Jul. 29, 2005, hereby incorporated by reference in its entirety. 

   BACKGROUND 
   When drilling in earth formations, various waste materials including drilling cuttings (i.e., pieces of a formation dislodged by the cutting action of teeth on a drill bit) are produced. Often, in circumstances where surface storage and disposal resources are limited, these waste products may be re-injected into the formation through a cuttings re-injection (CRI) operation. While the term “cuttings re-injection” is used to describe the operation, it should be understood by one of ordinary skill in the art that the term is used generically to describe any process whereby drilling waste including, but not limited to, drill cuttings, produced sands, water, scale, and other byproducts, are reintroduced into the formation using methods and apparatus described herein. 
   Typically, a CRI operation involves the collection and transportation of cuttings from solid control equipment on a rig to a slurrification unit. The slurrification unit subsequently grinds the cuttings (as needed) into small particles in the presence of a fluid to create a slurry. The slurry is then transferred to a slurry holding tank for conditioning. The conditioning process affects the rheology of the slurry, yielding a “conditioned slurry.” The conditioned slurry is pumped into a disposal formation by creating fractures under high pressure. Typically, the conditioned slurry may be delivered to the disposal formation through a casing annulus or a tubular system to a dedicated disposal wellbore but, in circumstances where such a wellbore is unavailable, the slurry may be delivered to a disposal section of a producing wellbore. The conditioned slurry is often injected intermittently in batches into the disposal formation. The batch process may involve injecting roughly the same volumes of conditioned slurry and then waiting for a period of time (e.g., shut-in time) after each injection. Each batch injection may last from a few hours to several days or even longer, depending upon the batch volume and the injection rate. 
   The batch processing (i.e., injecting conditioned slurry into the disposal formation and then waiting for a period of time after the injection) allows the fractures to close and dissipate, to a certain extent, the build-up of pressure in the disposal formation. However, the pressure in the disposal formation typically increases due to the presence of the injected solids (i.e., the solids present in the drill cuttings slurry), thereby promoting new fracture creation during subsequent batch injections. The new fractures are typically not aligned with the azimuths of previous existing fractures. 
   Release of waste into the environment must be avoided and waste containment must be assured to satisfy stringent governmental regulations. Important containment factors considered during the course of the operations include the following: the location of the injected waste and the mechanisms for storage; the capacity of an injection wellbore or annulus; whether injection should continue in the current zone or in a different zone; whether another disposal wellbore should be drilled; the required operating parameters necessary for proper waste containment; and the operational slurry design parameters necessary for solids suspension during slurry transport. 
   As many of the rigs used to drill oil and/or gas wells currently enjoy much smaller footprints than oil and/or gas wells of the past, the desired footprint for CRI operations has been reduced as well. As the CRI operation space has decreased, the need has arisen for space allocated to various pieces of equipment and systems to also decrease. Further, the decrease in available space and time spent preparing the site for CRI has accentuated the need for decreasing the footprint and preparation time for monitoring, as well as other associated equipment. 
   At locations where petroleum products are being recovered, refined or processed, a number of flammable gases may be present, including mixtures of oxygen, methane, ethane, propane, hydrogen sulfide and others. Standardized classifications for various types of hazardous locations have been adopted and assigned by regulatory agencies according to the nature and type of hazard that is generally present or that may occasionally be present. 
   Because electrical components, by their nature, may generate heat and sparks sufficient to ignite a flammable gas or other flammable mixture under even normal operating conditions, such components must be carefully designed, selected and installed when used in an area that is classified as hazardous. More specifically, the components must exceed certain minimum standards as to such characteristics as power consumption, operating temperature, current and voltage requirements, and energy storage capabilities. These standards are also established by regulatory authorities and vary depending upon the particular hazardous environment. 
   SUMMARY OF INVENTION 
   In one aspect, the claimed subject matter includes an apparatus to monitor properties of a solution to be used in an oilfield process including a flow loop in communication with a tank containing the solution, wherein the flow loop includes a pump, a viscometer and a densitometer. In one embodiment, the viscometer is configured to measure a viscosity of the solution and provide a viscosity output and the densitometer is configured to measure the density of the solution and provide a density output. In one embodiment, the apparatus includes a controller to receive the viscosity and density outputs and provide an operator interface terminal and system diagnostics, wherein the operator interface terminal is in communication with the controller and displays the viscosity and density outputs and system diagnostics. 
   In another aspect, the claimed subject matter includes a method to monitor properties of a solution to be used in an oilfield process, wherein the method includes communicating a tank containing the solution with a flow loop, wherein the flow loop comprises a pump, a viscometer, and a densitometer, pumping the solution from the tank through the flow loop, measuring a viscosity of the solution and outputting a viscosity reading with the viscometer, measuring the density of the solution and outputting a density reading with the densitometer, and evaluating the viscosity and density readings to determine the properties of the solution. 
   In another aspect, the claimed subject matter includes a method to inject a slurry into a subterranean formation, wherein the method includes measuring characteristic data from a well in communication with the subterranean formation, estimating downhole properties of the slurry using the measured characteristic data, measuring surface properties of the slurry with a measurement apparatus, determining optimal surface properties for the slurry from the estimated downhole properties, comparing the measured surface properties with the determined optimal surface properties, modifying the slurry until the measured surface properties are within tolerance values of the determined optimal surface properties, and injecting the modified slurry into the subterranean formation through the well. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective-view drawing of a re-injection system. 
       FIG. 2  is a perspective-view drawing of a skid-mounted monitoring system in accordance with embodiments of the present disclosure. 
       FIG. 3  is schematic layout of a flow loop in accordance with embodiments of the present disclosure. 
       FIG. 4  is a cross sectional drawing of a viscometer in accordance with embodiments of the present disclosure. 
       FIG. 5  is a block diagram of a re-injection monitoring system in accordance with embodiments of the present disclosure. 
       FIG. 6  is a schematic layout of a data management process in accordance with embodiments of the present disclosure. 
       FIG. 7  is a block diagram of a re-injection method in accordance with embodiments of the present disclosure. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present disclosure include methods and apparatuses to monitor the properties of a solution to be used in an oilfield operation. More particularly, selected embodiments describe methods and apparatuses to monitor the properties of a waste re-injection slurry prior to and during an operation to inject that slurry into a subterranean formation. 
   Referring initially to  FIG. 1 , an onshore cuttings re-injection site  100  is shown schematically. While re-injection site  100  is disclosed as an onshore system, it should be understood by one of ordinary skill that the systems described and disclosed herein are applicable to offshore, land-based, and remote (e.g., sub-sea, arctic, etc.) locations. In typical drilling operations, a mechanism  102  (e.g., one or more shale shaker screens) for removing solids and drill cuttings from the drilling fluid is provided. Next, the separated solids and cuttings are directed to a collection area  104 . A mixing tank  106  is also provided, in which the slurry to be injected is prepared. The waste solids are transferred from collection area  104  to at least one mixing tank  106  where salt water, fresh water, oily drains, production water, other fluids, and other components may be mixed therewith to create an injectable slurry. As the slurry is prepared, it is transferred to a holding tank  108  before being injected. Alternatively, CRI operations may utilize two (or more) mixing tanks  106  and  106 ′, wherein one tank  106  may prepare a slurry with coarse solids and the second mixing tank  106 ′ may prepare a slurry with finer solids. Either of the two slurries or a controlled combination thereof may be transferred to holding tank  108  before being injected into a well  110 . 
   Referring now to  FIG. 2 , one embodiment relates to a skid-mounted monitoring apparatus  10  to monitor various properties of a waste re-injection slurry. It should be understood by one of ordinary skill, that while the term “skid-mounted” is used to describe apparatus  10 , any configuration may be used. Particularly, the components of apparatus  10  may be confined to a single container (e.g., a skid) or may be spread out over a greater distance. Furthermore, apparatus  10  may be portable (i.e., moveable as a single unit), or may be configured in a more fixed, permanent configuration. As such, the physical size, configuration, and location of apparatus  10  is not to be limited by embodiments disclosed herein. 
   In this exemplary embodiment, the monitoring apparatus  10  depicted in  FIG. 2  includes a skid  12  to which a pump  14 , a viscometer  30 , and a densitometer  50  are mounted. Referring briefly to  FIG. 5 , a data acquisition control system  60  and operator interface terminal (OIT)  70  may be housed in a control system enclosure  62  in digital communication with equipment on skid ( 12  of  FIG. 2 ) and a plurality of sensors  80  located separately at the injection site. Optionally, in circumstances where monitoring apparatus  10  is located in a hazardous area, the OIT  70  may be remotely located and connected to remaining components on skid  12  via any networking or communication protocol known to one of skill in the art. 
   Referring now to  FIG. 3 , characteristics of the slurry are measured by various components of monitoring apparatus  10 . In one embodiment, skid  12  is positioned proximate to holding tank  108  in a location that minimizes the distance therebetween. Next, a viscometer  30  and a densitometer  50  of skid  12  are placed in fluid communication with the contents of holding tank  108 . As the slurry in holding tank  108  is prepared, the slurry viscosity and density characteristics are measured by viscometer  30  and densitometer  50  and analyzed before injection into the well. 
   As shown schematically in  FIG. 3 , skid  12  comprises a flow loop  15  to circulate the slurry mixture from holding tank  108 , through viscometer  30  and a densitometer  50 . An optional second densitometer (not shown) in series with densitometer  50  may be used for redundant measurement of the slurry through flow loop  15 . While flow loop  15  depicted in  FIG. 3  includes a single viscometer  30  and a single densitometer  50 , it should be understood by one of ordinary skill that any number of viscometers  30  or densitometers  50  may be used without departing from the scope of claims appended hereto. 
   As depicted in  FIG. 3 , flow loop  15  includes a plurality of lines  16 ,  18 ,  20 , and  22 , a plurality of valves  24 . Furthermore, at least one vent line  26  may be included to connect components of the flow loop  15  (e.g., viscometer  30  and densitometer  50 ) to a vent line of holding tank  108 . Alternatively, vent line  26  (if present) may be routed to an inlet of pump  14 . While one particular arrangement of flow loop  15  is depicted in  FIG. 3 , it should be understood by one of ordinary skill that any number of combinations or configurations may be used to connect viscometers  30  and densitometers  50  to slurry holding tank  108 . Generally, any combination of lines  16 ,  18 ,  20 , and  22  may be used in conjunction with various valve  24  configurations to direct the slurry in holding tank  108  through viscometer  30  and densitometer  50 . 
   Specifically, first line  16  communicates the slurry from holding tank  108  to pump  14 , wherein pump  14  is configured to circulate the slurry through viscometer  30  and densitometer  50 . Optionally, a strainer  28  may be located within first line  16  between holding tank  108  and pump  14 . Second line  18  communicates the pressurized slurry from pump  14  to viscometer  30 . Third line  20  communicates the slurry from viscometer  30  to densitometer  50 . Finally, fourth line  22  returns the slurry from densitometer  50  to holding tank  108 . At various locations within flow loop  15 , several valves  24  are positioned to direct and restrict flow of the slurry through flow loop  15 . 
   It should be understandable by one of ordinary skill that properties of the slurry will vary throughout the re-injection process and, thus, pump  14  may be selected to circulate a range of slurry viscosities and densities for extended periods of time. Further, as the slurry will, by its very nature, include particles of varying size and geometry, it is desirable for pump  14  to be durable enough to withstand wear and abrasion associated with pumping a slurry including such particles. Furthermore, in an effort to reduce damage to the measurement instruments, pump  14  may be configured to pump the slurry through densitometer  50  and viscometer  30  at reduced flow rates. These reduced flow rates may be dictated by physical limitations of the measurement instruments, or, they may be dictated by the type of measurement to be made. Thus, in one embodiment, a flow rate through flow loop  15  may be set not to exceed 20 gallons per minute. Furthermore, viscometer  30  is desirably positioned in close proximity to pump  14  along line  18  so that the distance through which the slurry must travel is minimized. Similarly, the pressure through line  18  may be controlled so that entrapped air is reduced. An excess of entrapped air may produce erroneous results or, in extreme cases, may damage flow loop  15  components. 
   Referring now to  FIG. 4 , viscometer  30  is shown constructed as a Couette-type viscometer employing a concentric cylinder geometry. While a Couette-type viscometer is shown for viscometer  30 , it should be understood that any type of viscometer, including, but not limited to, vibrating fork viscometers, funnel viscometers, and tube pressure drop viscometers may be used without departing from the scope of the claims appended hereto. Furthermore, in circumstances where multiple viscometers  30  are used, different type viscometers  30  may be deployed so that any advantages and disadvantages of each type may be accounted and compensated for. In a Couette-type viscometer  30 , an inner cylinder  32  is biased toward a preset position by a torsion element  34  located therein. A motor  36  provides rotation to a concentric outer cylinder  38  at a predetermined rotational velocity through a gear box  40 . In operation, the slurry is directed into an annulus  42  formed between outer cylinder  38  and inner cylinder  32 . As outer cylinder  38  is rotated, the slurry directed between inner cylinder  32  and outer cylinder  38  imparts a force to inner cylinder  32  causing rotational movement thereof. 
   The magnitude of the rotation imparted to inner cylinder  32  is a function of the resistive force of torsion element  34  and the viscosity of the slurry. Because the properties of torsion element  34  are known, the viscosity of the slurry may be determined. The measurement output of viscometer  30  is communicated through an output line  44  to data acquisition control system  60  (represented in  FIG. 5 ) and operator interface terminal  70 . It should be understood that such communication may be accomplished through any digital or analog communications protocol known to one of ordinary skill in the art. Furthermore, while it has been found that the viscosity of a slurry in an inline Couette-type viscometer  30  is more accurately measured when the rotational velocity of outer cylinder  38  is relatively slow, it should be understood that any of a range of speeds of outer cylinder  38  driven by motor  36  may be used. Particularly, the range of rotational velocity of outer cylinder  38  may be dictated by the physical constraints of the viscometer  30  used. Therefore, in one embodiment, the rotational velocity of outer cylinder  38  may be within the range of 0.1 to 60 revolutions per minute. While slow rotational speeds more accurately reflect the slow and no pumping conditions critical to CRI analysis, it should be understood by one of ordinary skill in the art that other, higher (or lower) may be used. 
   Because viscometer  30  is desirably mounted in close proximity with the equipment used to prepare and inject the slurry, and because the area in which the preparation and injection of the slurry takes place may be classified by relevant L standards as a hazardous area, viscometer  30  should be constructed as an explosion proof or intrinsically safe device as required by those standards. Both motor  36  and output line  44  utilize electrical current in some form, which may, in certain circumstances, become an ignition source. Thus, motor  36 , the interface between viscometer  30  and output line  44 , and output line  44  itself are preferably shielded, armored, and/or ventilated so as to meet requirements for local standards in hazardous areas. 
   Referring again to  FIG. 3 , at least one densitometer  50  is positioned such that it is in fluid communication with viscometer  30  through third line  20 . While densitometer  50  is depicted as a vibrating tube densitometer, it should be understood that other types of densitometer including, but not limited to, vibrating fork devices, Coriolis-type mass flow devices, magnetic devices, and radioactive devices may be used without departing from the scope of the claims appended hereto. Furthermore, in circumstances where multiple densitometers  50  are used, different types of densitometers may be deployed so that any advantages and disadvantages of each type may be accounted and compensated for. Preferably, densitometer  50  is in close proximity to viscometer  30  so as to minimize the distance the slurry must travel in third line  20  between viscometer  30  and densitometer  50 . As such, densitometer  50  operates to measure a flow density of the slurry and transmit data via an output line  52 . As described above in reference to viscometer  30 , it should be understood that such transmission may be accomplished by digital or analog communication through any of a variety of protocols. Furthermore, as mentioned above, an additional densitometer (not shown) may be provided in fluid communication with an outlet of densitometer  50 . Second densitometer may not be required, but may be included to allow for built-in redundancy in the event densitometer  50  becomes inoperable. Preferably, densitometer  50  is safe for use in areas that are zoned as hazardous. 
   Several valves  24  are located within lines  16 ,  18 ,  20 , and  22  of flow loop  15 . Valves  24  may be manipulated by data acquisition control system  60  to control the pressure and flow rate of the slurry therethrough. Gases entrained in the slurry are compressed and may be released through vent line  26  to holding tank  108  for treatment and venting, thereby maintaining pressure required to achieve accurate readings from viscometer  30  and densitometer  50 . 
   Referring to  FIG. 5 , control system  60  is shown housed within a control system enclosure  62  of monitoring apparatus  10 . Control system enclosure  62  may also house operator interface terminal  70  where an operator is able to monitor and modify the performance of monitoring apparatus  10 . Preferably, control system enclosure  62  is designed and constructed such that a sufficient amount of protection against exposure to drilling mud and other chemical agents is provided. It should be understood by one of ordinary skill, that while the control system  60  and enclosure  62  may be located within a hazardous area, operator interface terminal  70  may be located remotely, to an area outside the hazardous zone. 
   A plurality of remote sensors  80  are located at the injection site and are configured to measure and relay parameters including, but not limited to, flow rate, pump stroke, temperature, and pressure to control system  60 . Alternatively, remote sensors  80  may include outputs from additional viscometers and densitometers, if present. Control system  60  interfaces with pump  14 , valves  24 , viscometer  30 , and densitometer  50 , and receives data measured therefrom in addition to data transmitted by sensors  80 . As previously discussed, it may be necessary to modify the pressure and/or flow rate of the slurry through flow loop  15  in order to obtain more accurate readings. As such, control system  60  may interface with and actuate pump  14  and valves  24  to regulate the flow of slurry through lines  16 ,  18 ,  20 , and  22  of flow loop  15  to achieve the desired pressure and/or flow rate. Furthermore, parameters of the measurement devices, including, but not limited to, the rotational speed of outer cylinder  38  of viscometer  30  may be controlled. When desired, control system  60  may open and close individual valves  24  in concert with switching pump  14  off and on to purge and drain the components and lines of flow loop  15  for maintenance. Furthermore, valves  24  may be similarly manipulated to allow for rinsing or flushing of flow loop  15  with water or oil based fluids. Optionally, these operations may be performed automatically by the control system  60  based upon measurements provided from sensors  80  or from viscometer  30  and densitometer  50 . Alternatively, these operations may be performed manually either through an interface of control system  60  or by turning valves  24  installed in flow loop  15 . 
   Operator interface terminal  70  may display internal diagnostics of control system  60  including, but not limited to, current values and warning flags from remote sensors  80 , viscometer  30 , and densitometer  50 . It is contemplated that an operator may view real-time input values for parameters including well pressure, head pressure, pump stroke rate, slurry density, and slurry viscosity. Furthermore, the operator may view any or all input and output values, the status of the inputs and outputs, alarms, and controller health indicators from the operator interface terminal  70 . Additionally, troubleshooting and help information may also be provided to the user at the operator interface terminal  70 . As operator interface terminal  70  may be positioned on or near control system enclosure, it may be remotely located outside a hazardous area such that an operator may view and interact with it without having to enter the hazardous area. Additionally, when operator interface terminal  70  is located in an outdoor area, an adjustable sun visor (not shown) may be provided to remove glare from the display screen (not shown). 
   In circumstances where remote monitoring of the cuttings re-injection operation is desired, data may be transmitted from control system  60  via a server interface  90  to a location away from both flow loop  15  and control system enclosure  62 . In some circumstances, remote monitoring is desired because the flow loop  15  is located in a hazardous zone. In other circumstances, a single remote location is used to monitor several flow loops  15  of various cutting re-injection locations. Such a server interface  90  may be a personal computer or a processing device (e.g., a programmable logic controller) including a software application operable to receive data from control system  60  and provide the data in a format readable by the operator. 
   Further, a cuttings re-injection monitoring and diagnostic evaluation software module  94  may monitor parameters being measured by sensors  80 , viscometer  30 , densitometer  50 , at holding tank  108 , and at the well. Alarms may be initiated by the software module when measured values and/or derived parameters based on measured values fall below or rise above predetermined values or when a trend in the measured values and/or derived parameters indicates a potential issue. Furthermore, software module  94  may be in communication with a database  96  containing historical values and/or maximum and minimum values for such parameters monitored by software module  94 . While  FIG. 5  shows server interface  90 , software module  94 , and database  96  be contained within a single device  98 , it should be understood that separate devices connected by a communications network may also be used. 
   Alternatively, a remote operator interface  90 ′ may include a third party data acquisition system similar to that already in use by the operator. In this circumstance, control system  60  communicates data from sensors  80 , viscometer  30 , and densitometer  50  to remote operator interface  90 ′. Based upon the data provided, the operator may decide either to continue injecting the slurry having the properties measured or to modify the slurry in mixing tank  106  and/or holding tank  108  through the addition of solids, fluids, and additives. 
   Referring now to  FIGS. 5 and 6 , data collected from a particular re-injection site (e.g.,  100  of  FIG. 1 ) may be transmitted to a centralized data collection location  92 . This data transmission may be initiated by any of a variety of automatic or manual methods including, but not limited to, input by on-rig personnel, predetermined time schedules, accrued data quantity schedules, or by events triggered upon the diagnostic software configured to detect combinations of parameter values. As data is collected from various remote sites, it is loaded into a database management system for future reference. The data from any particular site may be reviewed by remote operator interfaces located at a re-injection site  90 A, at a support center  90 B, or at an administration location  90 C. Alternatively, data collected at a particular injection site may be transmitted directly to the centralized data collection area  92 . The data from a plurality of injection wells is collected and tabulated in the centralized data collection area  92 . 
   Analysis is performed on the collected data to develop profiles of different types of slurries used in various types of injection wells. The centralized data collection area  92  may include a secure administrative database. Using data provided by operators, potential risks may be identified at a single injection site based on deviation of measured parameters from control limits established from data collected from sites having comparable characteristics. In addition, advisories regarding preferred slurry characteristics may be made to the operator of a particular re-injection site based upon the comparison of that site&#39;s data to comparable data in centralized data collection area  92 . It is contemplated that data may be transmitted in real time to the centralized data collection area  92  for such analysis. The operator may then decide whether to inject the slurry having the current characteristics or return the slurry to mixing tank  106  or  106 ′ for modification of the slurry prior to injection. 
   In an alternative embodiment, monitoring apparatus  10  is used to monitor the slurry in one of the mixing tanks  106  or  106 ′. In this embodiment, the properties of the slurry are monitored as it is prepared. Based on the properties measured, additional solids or liquids may be added to the slurry until it exhibits the desired characteristics. The addition of solids, liquids, and/or additives may be automated, based on values obtained from the monitoring apparatus  10 . Manual control of the addition of slurry materials may be exclusive or shared with automated controls. 
   In another alternative embodiment, a first monitoring apparatus  10  monitors the slurry in holding tank  108  while a second monitoring apparatus (not shown) monitors the slurry being prepared in mixing tanks  106  or  106 ′. While systems in accordance with this embodiment require two monitoring apparatuses, they advantageously provide real-time data of the slurry both immediately prior to injection to the well and while still in mixing tank  106  or  106 ′. Such a monitoring system allows the slurry composition to be modified and monitored at the same time. 
   In another alternative embodiment, a first monitoring apparatus  10  is used to monitor the slurry in holding tank  108 , a second monitoring apparatus (not shown) is used to monitor the slurry being prepared in first mixing tank  106 , and a third monitoring apparatus (not shown) is used to monitor the slurry being prepared in second mixing tank  106 ′. In this embodiment, three monitoring apparatuses are used. As described above, real-time data pertaining to the slurry immediately prior to injection to the well is collected. Also, data at two mixing tanks  106  and  106 ′ may be used to determine whether, and to what extent, slurry characteristics should be manipulated by the addition of fluids, solids, and/or additives. 
   Additionally, the embodiments described herein may be used in conjunction with a slurry simulator to predict and/or measure the performance of a downhole cuttings re-injection operation so that real-time adjustments may be made to optimize the operation. Numerous variables, including, but not limited to, slurry temperature, slurry viscosity, slurry density, slurry particle size, injection pressure, injection flow rate, particle settling, borehole trajectory, and borehole geometry may affect the success and feasibility of a CRI operation. Particularly, in smaller boreholes in substantially horizontal trajectories, solids may rapidly accumulate at the bottom of the borehole and “stall” the re-injection operation. As a stalled condition may require remedial well intervention to be corrected, such stalling of the re-injection operation would be extremely costly. Furthermore, in circumstances where it is not feasible to measure certain variables (e.g., the temperature, and viscosity of the slurry downhole), the slurry simulator may be configured to estimate these values as a function of variables that are measurable (e.g., temperature and viscosity of the slurry at the surface, and the depth of the borehole). Therefore, in using a slurry simulator, various downhole conditions may be estimated and simulated to assist in modeling an “optimized” slurry that is more effectively injected downhole. Once such a model is created, the actual slurry may be measured and modified prior to injection to approximate the optimized model. 
   Of particular interest, a slurry simulator may be used to estimate the bottom-hole pressure as a function of time for a particular slurry. Often, CRI operations are performed in batches, whereby an amount of slurry is injected and the operation is paused when a predetermined pressure or amount of injected solution is reached. As time passes, the downhole properties, including the bottom-hole pressure of the slurry change until a stabilization point is reached. Once the stabilization point is reached, the CRI operation may continue to allow another amount of slurry to be injected into the formation. As the time to reach this stabilization point varies by slurry composition and wellbore properties, the ability to estimate the bottom-hole pressure and stabilization time of an injected slurry is of great benefit. Furthermore, through data analysis algorithms and historical methods, the slurry simulator may be capable of determining the bottom-hole pressure of an injected slurry as a function of properties (i.e., surface temperature and pressure) that are directly measurable. Using such analytical methods, a slurry simulator may be capable of outputting a real-time plot of bottom-hole pressure as a function of time for a particular re-injection well. As such, an operator of a CRI process can use such a plot to determine how large of a batch of slurry may be injected next, and when that injection may take place. 
   The slurry simulator may be either an analytical process or an apparatus capable of predicting the downhole behavior of the slurry. As such, the simulator may be based upon mathematical models (e.g., finite element analysis), a database of historical well data (e.g., as described above in reference to  FIGS. 5 and 6 ), or any other means for predicting performance. One slurry simulator that may be used in conjunction with embodiments of the present disclosure is described in U.S. patent application Ser. No. 11/073,448 entitled “Apparatus for Slurry Operation and Design in Cuttings Re-Injection” filed on Mar. 7, 2005 by Quanxin Guo and Thomas Geehan, hereby incorporated by reference in its entirety herein. 
   In using a slurry simulator, known values for certain variables are inputted so that unknown variables may be calculated or estimated. From these calculations, parameters for a theoretically optimal slurry are calculated. Next, using a measurement apparatus (e.g. apparatus  10  and flow loop  15  of  FIGS. 1-6 ), the state of the current slurry may be measured and compared with the optimal model to determine if the slurry may be modified to more closely approximate (i.e., fall within tolerances of) the optimal model. If changes are made, the measurement apparatus may again be used to verify the slurry composition before it is injected downhole. 
   Desirably, slurry simulator and measurement apparatus are operated in real-time in conjunction with one another to not only create an optimal slurry composition at the beginning of a CRI operation, but also to continuously re-evaluate the needs of the injected slurry and tweak its composition throughout the entire life of the CRI operation. Furthermore, while a single device may perform all the tasks of estimating, calculating, and optimizing, it should be understood that several devices may be used in conjunction with one another to accomplish the same goal. Additionally, it should be understood that as the properties of the injected slurry will certainly change as it is injected downhole, the slurry simulator may account for changes in slurry properties downhole when calculating the desired composition of slurry before injection. 
   Referring now to  FIG. 7 , a slurry injection method  200  is shown schematically. Preferably, slurry injection method  200  begins with the measurement of characteristic data from the well  202 . Next, properties of the formation and/or slurry that are not directly measurable are estimated or calculated  204 . For example, to calculate the temperature and pressure of the downhole formation and/or slurry, the measurable temperatures and pressures of a slurry or drilling fluid as it enters and exits the wellbore may be recorded. These differential pressure and temperature values may be used in conjunction with additional known or measurable quantities (e.g., well depth and temperature of the formation) to calculate the pressure and temperature of the slurry in the formation downhole. 
   Next, the slurry simulator uses the measured well characteristics in addition to the estimated and calculated downhole properties to determine the properties for an optimal slurry  206 . Next, the current properties of the slurry are measured  208  using a measurement apparatus (e.g., apparatus  10  and flow loop  15 ). If the measured slurry properties are within tolerances of the optimized slurry as determined by the slurry simulator  210 , the re-injection operation proceeds to inject the slurry  220 . If the measured slurry properties are outside of the optimized slurry tolerances  210 , the slurry is adjusted  212  and the measurement  208  and comparison steps  210  are repeated. Once brought within the tolerances of the optimal slurry, the slurry simulator may be continuously used to monitor the measured characteristic data and the surface slurry properties to make adjustments for changes in either the downhole formation properties or the surface slurry composition. Depending on the complexity of the slurry simulator and/or user interface, the slurry simulator may simply output an indication of “go/no-go” for the measured slurry or may output a complex graphical representation showing the where the slurry properties lie within the tolerance band. 
   Advantageously, embodiments described by the present disclosure allow for cuttings re-injection operations to be monitored and optimized for various configurations and types of re-injection wellbores. Using embodiments of the present disclosure, properties of waste and cuttings slurries can be monitored, modified, and optimized so that their re-injection into the formations can proceed as efficiently and cost effectively as possible. As a further advantage, a single slurry simulator may be capable of optimizing the slurry composition for several re-injection locations. As such, a single slurry simulator connected to various wellbore locations through a communications network may configure and optimize numerous re-injection wells with a minimal need for human presence in hazardous zones. 
   While the claimed subject matter has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the claimed subject matter as disclosed herein. For example, monitoring apparatus  10  may be used to monitor drilling fluids prepared for and used in a drilling operation. Accordingly, the scope of the claimed subject matter should be limited only by the attached claims. 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.