Patent Publication Number: US-8528394-B2

Title: Assembly and method for transient and continuous testing of an open portion of a well bore

Description:
SCOPE OF INVENTION 
     The present invention relates to testing of oil and gas wells. More specifically, the invention relates to an assembly and a method for transient and continuous testing of an open portion of a well bore. 
     PRIOR ART AND BACKGROUND OF THE INVENTION 
     The testing of oil and gas wells is of great importance for determining reservoir properties and production capacity of a hydrocarbon containing reservoir. Such testing is preferably made with a drill string, during so-called drill string testing (DST), during which a zone of interest is isolated by temporary packers, so that fluid from the reservoir zone may flow into the space between said packers. 
     U.S. Pat. No. 5,799,733 discloses a down-hole tool for early evaluation of a reservoir, primarily for taking samples of open-hole reservoir fluid. In said publication is described inflatable packer elements for isolating an open-hole reservoir interval of interest, a down-hole pump driven electrically or by a mud motor and providing a mud return to a drill string/test string or the annulus above the packers, and further are described a sample chamber and sensors for the measurement of fluid properties. Technology enabling an extended testing like the continuous mixing of mud and reservoir fluid during controlled conditions is however not disclosed, but several places give warnings against the risk for loss of pressure control, see for example column 16, lines 33-42 in U.S. Pat. No. 5,799,733. For embodiments having an electrically driven pump the formation fluid is fed to a well bore test string in order to eliminate the risk for loss of pressure control. For embodiments having a mud pump it is not possible to feed formation fluid into the upper part of a drill or test string, and for all such embodiments severe warnings are expressed against the risk of losing pressure control. 
     There is a demand for an assembly and a method for transient and continuous testing of an open portion of a well bore, without the above-mentioned limitations. 
     SUMMARY OF THE INVENTION 
     Aspects and preferred features of the present invention are set out in the appended claims. 
     The present invention is providing an assembly for transient and continuous, testing of an open portion of a well bore, said assembly being arranged in a lower part of a drill string, and is comprising:
         a minimum of two packers fixed at the outside of the drill string, said packers being expandable for isolating a reservoir interval,   a down-hole pump for pumping formation fluid from said reservoir interval,   a sample chamber,   sensors for measuring fluid properties,   a closing valve for closing the fluid flow from said reservoir interval, distinguished in that said assembly further is comprising:   sensors and telemetry for measuring and real-time transmission of flow rate, pressure and temperature of the fluid flow from said reservoir interval, from said down-hole pump in the drill string and in an annulus above the packers,   a mud driven turbine or electric cable for energy supply to said down-hole pump, and   a circulation unit for mud circulation from a drill pipe to said annulus above the packers and feeding formation fluid from said down-hole pump to said annulus, said circulation unit, independent of the circulation rate of mud to said annulus can feed formation fluid from said reservoir interval into said annulus, so that a well at any time can be kept in over balance and so that the mud in said annulus at any time can dissolve the formation fluid from said reservoir interval.       

     The present invention also provides a method for transient and continuous testing of an open portion of a well bore, employing the assembly according to the invention and arranged in the lower part of a drill string, whereby continuous testing is carried out by feeding formation fluid into the annulus above the packers isolating a reservoir interval, while transient testing is carried out by closing the formation fluid flow and measuring the response as a function of time, 
     the method comprising: 
     controlling said circulation unit based upon measured unit based upon measured data, the density and the reservoir fluid solubility of the mud, so that the well at any time is kept in over balance and said mud at any time can dissolve the reservoir fluid fed into said mud. 
     The present invention enables the testing of the production properties of a reservoir without using surface process equipment. Well testing is carried out in an open hole without the use of casing, meaning saving time. Further testing can be done independently in an unlimited number of test zones without having to trip in and out of the well bore, which gives a considerable cost and time saving. There is no need for conventional sub-surface test equipment for providing well control. Open-hole testing is possible without limitations regarding flow rate and duration. The pumping of reservoir fluid from a reservoir to the well can be done at a high flow rate, at great pump capacity, with large quantity of mud dissolved, which opens for testing of high permeability reservoirs. The testing is carried out in an open well and having all well control barriers in place, that is having weighted mud in the drill string and annulus at full over balance, as well as blow-out preventer (BOP) and down-hole closing valve above the packer elements. Preferably the assembly comprises a connection line for pressure communication over/under packer(s) to maintain the hydrostatic pressure, which means over balance, in the entire open hole. The assembly is preferably adapted for reducing well related noise and improve the differential pressure specifications, in particular by preferably using double packers over/under the test zone. Reservoir fluid is pumped out utilizing an electric or hydraulically driven pump. When using an electrical driven pump the pumping is always undertaken so as to provide a sufficient thinning or a complete dissolving of reservoir fluid in the drilling fluid by adjusting the flow rate so as to maintain a stable well, even during circulation stop. When using a hydraulically driven pump hydraulic energy is transformed to electric energy driving a hydraulic pump via a mud circulation turbine and generator. Alternatively, the hydraulic pump is driven by a hydraulic circuit in turn driven by a hydraulic mud circulation turbine, or a mud circulation turbine drives an electric pump. The flow rate thereby can be adjusted so that a stable well is maintained, even during circulation stops, independent of whether the pump is driven electrically or hydraulically. By controlling the input pumping of formation fluid based upon measured data, the mud density and the reservoir fluid solubility of the mud, the well thereby can be kept in over balance at any time and the mud can at any time dissolve the reservoir fluid fed into the mud. 
     The assembly comprises sensors for the measurement of chemical and physical properties of produced reservoir fluid, preferably chosen amongst sensors for or based upon optical spectroscopy, pH resistivity, gas/oil ratio, viscosity, and other sensor types known to the art. Additionally, the assembly comprises pressure and temperature meters for measuring pressure and temperature in the test zone, that is reservoir pressure and temperature, as well as the pressure and temperature in the pump, drill string and the annulus volume. The assembly comprises a circulation unit that is a flow diverter enabling controlled mud circulation from drill pipe to annulus at the same time as reservoir fluid from the down-hole pump is mixed with and dissolved in the mud, which makes it possible to produce a large volume of reservoir fluid without risking under balance or uncontrolled entering of reservoir fluid to the well. The assembly further comprises means for down-hole rate measurement and flow control. Further, the assembly comprises a closing valve that makes it possible to have an accurate closing of the well flow for the measuring of pressure response from the reservoir, that is transient testing. The assembly also comprises advantageously a telescope unit to take up expansion and contraction of the drill string or a set production packer (important for preventing displacement of packer elements and noise in pressure meters in the well test phase). The drill string comprises preferably a drill bit at the end of the assembly for hole conditioning before, between and after the formation testing. Natural gas coming from the mud/hydrocarbon solution at the return to the surface is fed through the mud conditioning equipment of the drilling installation and is vented to the air. Dissolved oil is accumulated in the mud and is left in the well in connection with the permanent return plugging after finished testing. Possible surplus mud can either be transported for destruction or reinjected to the reservoir. The present assembly and method advantageously make use of mud having a high solubility for reservoir fluid. 
    
    
     
       THE DRAWINGS 
       The present invention is illustrated by drawings, of which: 
         FIG. 1  illustrates an assembly according to the invention, 
         FIG. 2  illustrates an alternative assembly according to the invention, 
         FIG. 3  illustrates a sampling chamber for use together with the assembly and the method according to the invention, 
         FIG. 4  illustrates a sampling chamber for use together with the assembly and the method according to the invention, and 
         FIGS. 5 to 11  illustrate a sequence employing the assembly and the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     By the present invention open-hole testing is enabled, without using down-hole valves and surface processing equipment, while having unlimited flow time, unlimited flow volume and unlimited duration of closing. The features defined in the present claims make it possible to obtain such expanded flow rate and test duration without the risk for uncontrolled well blow-out. 
       FIGS. 1 and 2  show two embodiments of the assembly according to the invention. In the embodiment of  FIG. 1  pumped in reservoir fluid and circulated mud are introduced at the same level in the annulus over the packers, whereas the embodiment according to  FIG. 2  illustrates introducing circulated mud and pumped in reservoir fluid into the annulus over the packers at different levels, as the circulation unit is arranged in a divided version. Other embodiments are also conceivable, but in any case the circulation unit is arranged so that circulated mud and pumped in formation fluid can be fed to the annulus over the packers under full control regarding the maintaining of overbalance and dissolving all the pumped in formation fluid in the mud. 
     Shown in  FIGS. 1 and 2  are a drill pipe  1 , a slip joint  2 , a pump  3 , a pump outlet  4 , DFA tools and sample chambers  5 , a probe  6 , a straddle packer  7  with pressure gauge, a drill bit  8 , shale  9 , sand  10 , and a flow diverter  11 . 
       FIGS. 3 and 4  present a further illustration of a down-hole fluid analyser and a sample chamber (DFA). 
     The following are associated with the sample chambers of  FIG. 3 : Thin walled chamber ˜75 liters/9 meters; Packaged in the 7″ OD sleeve to provide circulation path; ‘Smart Piston’, self closing; Pressure release valves; Stackable; Hydraulic and electrical lines pass through/around chamber. 
     Shown in  FIG. 4  are DFA &amp; sample chambers  15 , a 7 inch OD flow sleeve  12 , centralisers  13 , a tool wiring harness  14 , and a sample flow line  16 . 
       FIGS. 5 to 11  illustrate a drilling operation and a test carried out using a drill string having an assembly according to the invention. The sequence illustrated in  FIGS. 5 to 11 , with some explanatory text below, is self-evident for the persons skilled in the art. 
     Shown in  FIG. 5  are a top drive  17 , BOP  18 , sea bed  19 , cased hole  20 , and open hole  21 . Shown in each of  FIGS. 6 to 11  are a top drive  17 , BOP  18 , and sea bed  19 . 
     Job Sequence  1  associated with  FIG. 5  is as follows: Drill Well to TD; Perform openhole logging; RIH with FTWT; Circulate through the drill bit on bottom. 
     Job Sequence  2  associated with  FIG. 6  is as follows: Fix tubing in BOP and Inflate FTWT packers 
     Job Sequence  3  associated with  FIG. 7  is as follows: Circulate above top packer. 
     Job Sequence  4  associated with  FIG. 8  is as follows: Isolate active mud system and pump out formation fluid from between packers to the annulus while continuing circulation with return through the kill and choke line through degasser. 
     Job Sequence  5  associated with  FIG. 9  is as follows: Stop circulation→Stop pumping out reservoir fluid; Measure pressure build up between packers for transient analysis. 
     Job Sequence  6  associated with  FIG. 10  is as follows: Circulate above top packer; Perform formation integrity test (optional). 
     Job Sequence  7  associated with  FIG. 11  is as follows: Deflate FTWT packers; Open BOP to unlock tubing; Circulate through the drill bit to condition well; Pull out of hole or go to next test zone.