Method of in-situ testing of a drilling fluid

Abstract: The method comprises during a drilling operation wherein the drilling fluid is set moving and the drill string is stationary, monitoring the pressure of the drilling fluid pumped into the drill string depending on the volume of liquid pumped in the drill string and determining, from the pressure curve, a physical property linked to the thixotropy of the drilling fluid. An advantage of the invention is that the highest point of the pressure curve indicating the start of the fluid flow into the well is easily visible, and its maximum value can be measured to find the gel strength specific value.

Description: This invention relates to a method of in situ testing of a thixotropic drilling fluid during drilling of a well using a drilling tool with a drill bit and drill string formed from drill pipes joined together. In the rotary drilling of an oil or geothermal well a drill string is formed from a set of pipes joined together and a drill bit fitted at one end. The drill bit drills the rock when it starts rotating, either by rotating the drill string from the surface or by using a hydraulic motor situated above the drill bit. A drilling fluid, normally called "mud", is pumped from the surface inside the drill string, goes through the drill bit, and comes back to the surface through the annulus existing between the wall of the well and the drill string. Mud is an important part of the drilling process, and is used for several purposes. One of them is to create hydrostatic pressure on the drill bit sufficient to counterbalance the pressure of the fluids present in the rocks which are being drilled. This hydrostatic pressure must not be so high as not to fracture the rock. The density of the mud must be maintained between minimum and maximum values. Another function of the mud is to bring back to the surface the rock cuttings which have just been drilled. For this the mud viscosity must be sufficient to keep the cuttings suspended. However, viscosity cannot be so high as to prevent pumping and circulating of the drilling fluid in the well. In use, the drilling fluid is either stationary, and has a tendency to gel, or is circulated by means of a pump from the surface to the inside of the drill string and rises towards the surface in the annulus between the wall of the drilled well and the drill string assembly. Every time drilling progresses in depth by one drill pipe length, fluid circulation must be stopped while another pipe is added to the drill string. During this operation the drilling fluid which is stationary in the well contains the cuttings that the fluid is bringing to the surface. To prevent these cuttings from going back to the bottom of the well a thixotropic fluid is used. The rheological properties of the mud are affected by the drilling conditions, such as the temperature in the well and the types of rocks drilled. As an example, when drilling a clay formation the clay dissolves in the fluid, increasing greatly the mud viscosity and the yield stress. It is therefore essential to test and control the drilling fluid's properties so as to be able to modify its formula either to maintain a chosen formula or modify it depending on the drilling conditions. Normal practice on drilling sites is to take a sample of mud regularly and test its rheological properties, especially its viscosity. However, these test conditions are not equivalent to the conditions prevailing in the well, and do not necessarily reflect accurately the state of the mud being used. This method is described in US Patent 4,726,219 and GB Patent 1,280,227. A method of in situ testing of the rheological properties of drilling fluids is described in the Article "Surface recorder can signal downhole drilling problems" in World Oil (November 85 p71-77). However, the rheological properties of a drilling fluid can only be tested when the mud is circulating. This invention proposes a method of in situ testing of the drilling fluid which avoids the drawbacks of previous methods. To be more precise, this invention provides a test method for a thixotropic drilling fluid during drilling operations carried out with a drilling tool including a drill bit, a drill string assembly formed from drilling pipes joined together. The drilling fluid when stationary has a tendency to gel but is fluid when the drill string is being rotated or the drilling fluid is being circulated by means of a pump from the surface to the drill bit inside the drill string and rising towards the surface in the annular space provided between the wall of the well already drilled and the drill string. When the circulation is restarted the drilling tool is stationary; the evolution of the pressure of the fluid being pumped in the drilling tool can be monitored. One aspect of the invention is to be able to monitor the pressure peak corresponding to the start-up of fluid circulation in the well, and to measure its maximum value so as to find the gel strength of the gelled mud. In particular, the present invention provides a method of determining in situ the gel strength of a thixotropic drilling fluid during the drilling of a well using a drill string assembly including a drill bit and drill pipes joined together, the drilling fluid being in operation circulated by means of a pumping unit from the surface to the drill bit inside the drill string and rising to the surface through the annular space existing between the wall of the well already drilled and the drill string, the method being characterised by stopping the pumping unit, and thus the circulation of the drilling fluid, and allowing the fluid to gel, and then re-starting the pumping unit after the fluid has gelled, monitoring the evolution of the pressure of the drilling fluid at the outlet of the pumping unit and determining the pressure peak having the maximum pressure before the drilling fluid starts to re-circulate through the well, the difference between this pressure peak value and the asymptotic pressure value immediately following the pressure peak being representative of the gel strength of the drilling fluid. A further aspect of the invention resides in the possibility of determining the yield strength and the compressibility of the gelled mud from the rising part of the pressure peak. When the drilling tool starts rotating, if the evolution of the fluid pressure is monitored, two values of the physical properties can be obtained: one dynamic when the drilling tool is rotating, and the other static when the drilling tool is stationary. A yet further aspect resides in the possibility of determining the asymptotic value of the down curve of the pressure peak. From this asymptotic value the pressure drop due to fluid loss in the well can be determined. The operation can be repeated to follow the evolution of the pressure of the fluid being pumped. This operation can be repeated almost every time that a drill pipe is added. The successive evolutions of the pressure can be compared, and the variations of the physical properties characteristic to the thixotropy of the drilling fluid can be found. The invention will be better understood when reading the following description and the attached Figures, in which: Figure 1 shows a sketch of a well being drilled and the surface equipment used for circulating and cleaning the drilling fluid; Figure 2 a shows a rheogram of the mud i.e. the shear stress ST , the shear rate SR and Figure 2 b represents the evolution of pressure p of the fluid being pumped in relation to the volume of the pumped fluid for different levels of mud gelation; Figure 3 shows three diagrams, in terms of time, the number of pump cycles N , the flow rate Q and the pressure p of the pumped fluid when the drill pipe is being added; Figure 4 shows the evolution of pressure p of the pumped fluid in relation to the number of pump cycles, drawn from Figures 3 a and c ; and Figure 5 shows the evolution of pressure p in relation to the number of pump cycles N drawn from Figure 5 a and c . Figure 1 shows a schematic of a drilling well (10) with a drill string (12) including drill pipes (14) and a drill bit (16). A drilling tower (18) allows handling of the drill string from the surface, particularly to add pipes to the drill string and to start rotating the drill string (16) to drill the rock. The drill bit rotation can also be carried out with a motor situated at the bottom, particularly when drilling deviated wells. Every time the well is drilled for an additional depth of a pipe length, about 9 metres, a new pipe is added to the top end of the drill string on the surface. The drilling of the well will start again until another length of pipe is drilled. This is repeated until the drill string is removed from the well either because the drill bit is worn or because the desired depth has been reached. A drilling fluid, generally called "mud", is kept in a mud tank (20). This fluid is circulated by a pump (22). The fluid passes up a rigid pipe (24), then a standpipe (26), before being sent into the drill string from an injection head (30) connected to the standpipe (26) by a flexible pipe (28). The first pipe (34) connected to the injection head (30) has a square section so that it can be rotated from a rotating table (not shown). The drill pipes added one after the other during drilling operations are fitted between the square pipe (34) and the drill string (12). The drilling fluid circulates inside the drill string (12), then through the drill bit (16) via the injectors up to the surface in the annular space (36) existing between the drill string and the wall of the well (10). At the surface, mud goes through a cleaning process (38) in which the cuttings (40) are separated from the mud [which then returns through pipe (42) in the mud tank (20)]. New mud and/or adjuvants can be added in the tank through pipe (44). The cuttings (40) are sent through the pipe (46). The pumping equipment includes a sensor (48) recording pump cycles. Each pump cycle corresponds to a certain volume of fluid pumped in pipe (24). The number of cycles allows the determination of the volume of fluid pumped inside the drill string. A flow rate valve placed inside pipe (24) could be used instead of sensor 48 to measure the volume of fluid pumped inside the drill string. A pressure sensor (50) situated between pump (22) and the injection head measures the pressure of the fluid pumped inside the column. Sensors 48 and 50 are connected to a data recorder (52). This recorder allows, for example, real time recording of the evolution of the pressure measured by sensor (50), as well as the number of pump strokes detected by sensor (48). This recorder also allows there to be measured the evolution of the pressure related to the number of pump cycles. One of the main functions of the drilling mud is to carry the cuttings produced by the drill bit from the bottom of the well to the surface through the annular space (36). Every time a drill pipe is added to the drill string (40), pump (22) is stopped and circulation of the mud is also stopped. When the mud is stationary, the cuttings present in the annular space have a tendency to fall to the bottom of the well. In order to prevent such an inconvenience, a relatively viscous drilling fluid is used to maintain the cuttings in suspension when the fluid is stationery. However, the viscosity of the mud cannot be too great else the pumping means will be unable to circulate the mud effectively in the well. This is dealt with by using a thixotropic drilling fluid - that is to say, a fluid in which the viscosity decreases when the fluid is placed in rotation or agitated. It is current practice, in order to find the fluid behaviour, to trace a rheogram showing the shear stress ST as compared with the shear rate SR applied to the fluid. This is shown in Figure 2 a . For this, a viscosimeter is used to submit the fluid being tested to a given shear rate and record the shear stress. The viscosimeter most often used in the Petroleum Industry is the FANN viscosimeter. It has two coaxial cylinders between which is placed a mud sample to be tested. The mud shear stress is obtained by rotating one cylinder against the other; the shear stress is then defined by the strength necessary to apply to the other cylinder to stop rotation. Another type of viscosimeter is made of a narrow tube in which a mud sample circulates. The pressure difference is recorded (p₁ - p₂) between the entry and exit of the fluid in and out of the viscosimeter as a function of flow rate Q . For this type of viscosimeter, the shear stress is given by: ST = D (p 1 - p 2 )/4L D and L being respectively the diameter and the length of the viscosimeter. The shear rate SR is given by : SR = 32Q/3,14 D 3 The rheogram of Figure 2 of the shear stress ST against the shear rate SR is equivalent to a diagram showing the variation of the fluid pressure in relation to flow rate Q , knowing the shape of the tube in which the fluid circulates. The rheogram of Figure 2 is typical of a non-Newtonian fluid; to activate this fluid it is necessary to submit it to a minimum shear stress ST₀ , called the yield stress. With a shear stress higher than ST₀ , the fluid is circulating. The slope of the curve ST against SR is, by definition, the apparent viscosity of the fluid. However, for thixotropic fluids such as drilling mud which have a tendency to gel when stationary, the shear rate ST necessary to activate the fluid is higher than the yield stress ST₀ . This shear stress, called the gel strength, is indicated by point A on the rheogram of Figure 2 a . When the gel strength of the gelled fluid is reached, the shear stress decreases rapidly down to point B to follow the curve shown in Figure 2 a . In this invention, when the circulation of the fluid is started again with the pumping unit the evolution of the pressure of the fluid pumped in the drill string in relation to the number of pump cycles can be clearly seen, taking into account the volume of the fluid pumped in the drill string and with the drilling fluid being stationary at the beginning of the experiment. The pressure curve reaches a maximum at gel breaking point - i.e. at the gel strength of the gelled fluid. This defines the physical property of the thixotropy of a drilling fluid. In good conditions, this pressure test is carried out after having added a pipe to the drill string when circulating by pumping is resumed. If this test is carried out regularly, and if the period during which the fluid remains stationary is kept constant, it is possible to follow the evolution of the physical property of the drilling fluid thixotropy, and particularly the evolution of the gel strength, during the fluid's life in the well. Figure 2 b shows the evolution of the fluid pressure measured by sensor (50) from the number of pump cycles N of pump (22) measured by sensor (48) with the fluid being stationary. The curve (60) shows the evolution of the pressure for a non-gelled fluid. The curve reaches its asymptotic value p a showing the pressure drop in the drill string and in the annulus corresponding to the smallest flow rate of the fluid. The curve (62) shows the evolution of the pressure related to the number of pump strokes N , for a gelled fluid and the resumption of circulation. The drill string is stationary. The pressure reaches a peak (64) when the number of pump strokes is equal to 8, when a certain amount of fluid is injected in the drill string. Before reaching this peak the gelled fluid is stationary. When maximum pressure is reached, the gel breaks, and pressure drops rapidly (curve 66) to reach the asymptotic value p a . The highest pressure p m corresponds to the gel strength of the gelled fluid The maximum value varies from the degree of gelation of the gel, which increases rapidly when circulation of the fluid stops to reach a stabilised value after a while. To compare the gel strength of two types of fluids, or to follow the evolution of the gel strength of a gelled fluid during its utilization, the successive pressure tests (curve 62, Figure 2 b ) must be done while the fluid is stationary during a relatively constant period of time before each test. The rising part between N=0 and N=8 shows the elasticity and compressibility of the gelled fluid. Curve (68) shows the evolution of the pressure for the same gelled fluid as in curve (62) but the drill string is rotating at more or less constant speed. If the rotation speed of the drill string is fairly low, and the fluid inside the drill string is considered as turning together with the drill string when the fluid in the annulus is agitated, the gel in the annulus is broken. The difference in the pressure indicated in (70) in Figure 2 b is then the gel strength of the gelled fluid in the drill string. The difference of pressure p m -p a , indicated at (72), indicates the static gel strength of the gelled fluid in the drill string and in the annulus. The following Figures illustrate the invention with measurements taken during drilling operations. The diagrams of Figures 3 and 5 were recorded as per time and indicated in seconds. The pump is started again at time t₀ at slow speed with a small flow. From time t₁ the number of pump strokes increases. Figure 3 b , which shows the flow Q in relation to time, is no less than the integral of the number of pump cycles of Figure 3 a in relation to time. The flow is indicated in litres per minute. Between time t₀ and t₁ the flow Q is small and constant. It increases rapidly at time t₁ to reach stabilisation at a relatively constant value. In Figure 3 c , it can be seen that pressure p , indicated in MPa, goes to a maximum (80) between time t₀ and t₁ . This maximum (80) is the yield point of the gelled fluid. Pressure then rises rapidly, to stabilise at a relatively constant value. Figure 4 shows the evolution of the pressure p of the pumped fluid related to the number of pump cycles N . The curve was made by combining Figures 3 a and 3 c . Pressure is relatively stable around 1 MPa until a number of pump cycles of around 10. This number of pump cycles corresponds to the volume of fluid necessary to inject in the drill string to compress the air sent into the drill string when a pipe is added. A pressure peak (82) happens, shown by a rapid increase of pressure (84) followed by a drop (86) until a number of pump cycles N of 22. Then, pressure increases rapidly (part of curve 88) until it stabilises. The maximum (82) of the pressure peak corresponds to the breaking point of the gelled fluid or its gel strength. As long as maximum (82) of the pressure has not been reached, the fluid remains stationary in the well. It only starts circulating again when maximum (82) is reached. If the driller had not increased the pump's flow from the number of cycles N =22, the pressure drop (86) would have stabilised until reaching a plateau (90). The data in Figure 5 was recorded during the same well as Figure 3, and with the same type of drilling fluid, but two and a half hours later. Figures 5 a , b and c show respectively the number of pump cycles N , the flow Q in litres per minute and the pressure p in MPa, recorded as per time t . The pump is restarted at time t₀ . In Figure 5 b the successive flow rate in seconds are indicated between time t₀, t₁, t₂ and t₃ . On Figure 5 c , a pressure peak (92) appears at time t₀ . The curve of Figure 6 showing the evolution of the pressure in relation to the number of pump cycles was obtained by combining the Figure 5 a and 5 c curves. In Figure 6, pressure is relatively constant, at 1 MPa, until the number of pump cycles equals 15. This part of the curve shows the air being compressed in the drill string. The pressure then increases rapidly, curve (96), until a maximum value of 4 MPa for a number of pump cycles equal to 20. This rise in pressure indicates the elasticity and compressibility of the gelled fluid. The maximum of the pressure, indicated at (94), is the gel breaking point and the moment from which the fluid is recirculated in the well. The pressure then drops to an asymptotic value of approximately 3 MPa. The difference between maximum value of 4 MPa and the asymptotic value of 3 MPa is the static gel strength of the fluid gelled at 1 MPa. Between time t₂ and t₃ the pump flow is changeable. After time t₃ pressure increases rapidly. The comparison between pressure peaks (82) (Figure 4) and (94) (Figure 6) allows the definition of the changes of the thixotropic properties of the drilling fluid in relation to time. The peak maximum values allow the comparison of the different gel strengths of the gelled fluids, the asymptotic values [(90) in Figure 4 and (98) in Figure 6] allow the comparison of the loss of fluid in the well, and the differences between the peak maximum values and the asymptotic values allow the definition of the changes in the static gel strength of the gelled fluid. The pressure rises shown at (84) in Figure 4 and (96) in Figure 6 allow the evolution of the elasticity and compressibility of the gelled fluid to be followed. The found values, such as the gel strength of the gelled fluid, can be compared both one against the other and also be compared against a predetermined value. If, for example, the gel strength of the gelled mud must not exceed a set value, and if the measurements done with this invention show that the value has been exceeded, or is going to be exceeded, the mud formula can be modified to bring the mud properties to the planned specifications. If necessary, changes can be made to allow for the increase in the drill string length as pipes are gradually added.

Claims: A method of determining in situ the gel strength of a thixotropic drilling fluid during the drilling of a well (10) using a drill string assembly (12) including a drill bit (16) and drill pipes (14) joined together, the drilling fluid being in operation circulated by means of a pumping unit (22) from the surface to the drill bit (16) inside the drill string (14) and rising to the surface through the annular space (36) existing between the wall of the well (10) already drilled and the drill string (12), the method being characterised by stopping the pumping unit (22), and thus the circulation of the drilling fluid, and allowing the fluid to gel, and then re-starting the pumping unit (22) after the fluid has gelled, monitoring the evolution of the pressure of the drilling fluid at the outlet of the pumping unit (22) and determining the pressure peak (64) having the maximum pressure before the drilling fluid starts to re-circulate through the well (10), the difference between this pressure peak value (64) and the asymptotic pressure value immediately following the pressure peak being representative of the gel strength of the drilling fluid. A method as claimed in Claim 1, wherein the pressure is separately measured first when the drill string (12) is stationary (62), to give a static gel strength value, and then when it is rotating (68), the rotation speed of the drill string being set such that the drilling fluid inside the drill string circulates together with the drill string and yet the drilling fluid within the annular space (36) is agitated so that the gel is broken, to give a dynamic gel strength value, thereby to indicate the values of the gel strength first for all the drilling fluid in the well and second for the drilling fluid within the drill string only, from which values there may be calculated by simple subtraction a value indicative of the gel strength of the drilling fluid in the annular space. A method as claimed in either of the preceding Claims, wherein the measurements are repeated regularly, after having added a drill pipe, to identify any changes in the physical gel strength.

Inventors: BURGESS TREVOR MICHAEL, BURGESS, TREVOR MICHAEL
Applicants: SCHLUMBERGER SERVICES PETROL, SERVICES PETROLIERS SCHLUMBERGER, (FORMERLY SOCIETE DE PROSPECTION ELECTRIQUE SCHLUMBERGER)
Application year: 1990
Main classification: E21B21
IPC codes: E21B21