RHEOLOGICAL PROPERTIES

Background

[0001] As a borehole is drilled into a production formation, drilling fluid is pumped down a borehole through a drill string to power the drill bit at a cutting end of the drill string. The drilling fluid then exits the drill bit to convey drill cuttings up and out of the borehole. As the drilling fluid flows up on the outside of the drill string, the pressure and chemical composition of the drilling fluid can interact with the formation. For example, over-pressurizing the drilling fluid can fracture the formation, and under-pressurizing can allow cave-in, among other potential interactions. Therefore, it is helpful to know the pressure at points along the length/depth of the borehole.

[0002] Collecting real-time pressure at all points along the borehole can be difficult due to the many moving parts and the constant lengthening of the borehole. Models may be used to predict the pressure in the borehole, but the models rely on a broad range of characteristics of the drilling fluid, such as rheological properties at a variety of temperatures and pressures, to achieve accurate results. Measuring all the characteristics at the various temperatures and pressures is impractical in real-time since the characteristics are changing constantly as additional drill cuttings are produced and mixed in to the drilling fluid.

Brief Description of the Drawings

[0003] FIG. 1 is a schematic and cross-sectional diagram of an embodiment of a drilling system using a well modeling system;

[0004] FIG. 2 is a list of possible temperatures and pressures that a well modeling system may select from to test and record the viscosity dial readings;

[0005] FIG. 3 is a list of data sets that were recorded by a well modeling system; and [0006] FIG. 4 illustrates a shear stress vs. shear rate function for viscosity dial readings that is generated for the full range of pressures and temperatures.

Detailed Description

[0007] Turning now to the figures, FIG. 1 is a schematic and cross-sectional diagram of an embodiment of a drilling system 100 using a well modeling system 138. The drilling system 100 is for drilling a borehole 106 using a drilling fluid, and includes a drilling rig 102 located at the earth’s surface 104 of the borehole 106. The drilling rig 102 provides support for a drill string 108. The drill string 108 conveys the drilling fluid from the surface 104 to a bottom-hole assembly 110 through a drill pipe 112. The bottom-hole assembly 110 has a drill collar 114, a downhole tool 116, and a drill bit 118. Other systems 100 may include additional or alternative components in the bottom-hole assembly 110.

[0008] The drilling system 100 pumps drilling fluid through the drill pipe 112 to power the downhole tool 116 and the drill bit 118. The drill collars 114 may be used to add weight to the drill bit 118 and stiffen the bottom-hole assembly 110. The downhole tool 116 may comprise any of a number of different types of tools including measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, pressure sensors, temperature sensors, and others. The bottom-hole assembly 110 creates and logs the borehole 106 by penetrating the surface 104 to access subsurface formations 122.

[0009] During drilling operations, a mud pump 124 pumps the drilling fluid (sometimes known as“drilling mud”) from a mud pit 126 through a hose 128 into the drill pipe 112 and down to the drill bit 118. The drilling fluid flows out from the drill bit 118 and returns to the surface 104 through an annular area 130 between the drill pipe 112 and a side 120 of the borehole 106. The mud pump 124 can be controlled to influence the speed and effectiveness of the drill bit 118 pumping the drilling fluid fast enough to remove the cuttings that are drilled, cool the drill bit 118, and lubricate the drill string 112. Faster pumping increases the effectiveness of transporting the cuttings, but the mud pump 124 will not pump as efficiently.

[0010] Generally, the drill bit 118 will drill faster with a higher pressure from the mud pump 124. The benefits of higher pressure can reach a limit, however, because the drilling fluid, among other things, exerts a pressure against the side 120 of the borehole 106. If the mud pump 124 pressurizes the drilling fluid too much, then the pressure against the side 120 can cause drilling fluid to penetrate into the subsurface formations 122. Furthermore, the drilling fluid can fracture or break down the side 120 and lose fluid to the formation. Too little pressure can also be a problem in the borehole 106, since fluid and gases from the formations 122 can come into the borehole 106 and expand to the surface 104. In summary, drilling with a pressure that is too high or too low can damage the subsurface formation 122, or otherwise decrease the fluid flow from the subsurface formation 122. Thus, it is useful to know the pressure at locations within the borehole 106.

[0011] The pressure of drilling fluid is easier to model when the fluid is stationary, since the density of the fluid is so highly determinative under those conditions. Unfortunately, during a drilling operation the drilling fluid is in constant motion. Despite being in constant motion, a well modeling system can determine the downhole pressures if values for rheological properties of the drilling fluid are accurately known and included in the models. Rheological properties for the drilling fluid can be measured at the surface, but these rheological properties change as the drilling fluid cycles through the borehole 106, the mud pit 126, the hose 128, and the drill pipe 112, etc. Drill cuttings that are not filtered out of the drilling fluid can change the chemical makeup and rheological properties of the drilling fluid. Furthermore, the formations 122 may include chemicals that dissolve into the drilling fluid and further change the makeup of the drilling fluid and how additional interactions with the formation 122 will influence the side 120. It is thus important to consistently update the rheological properties of the drilling fluid throughout the drilling process.

[0012] To monitor the rheological properties and model the pressure of the drilling fluid in the borehole 106, the system 100 includes a well modeling system 138. The well modeling system 138 receives information from a rheometer 142 that collects samples of the drilling fluid and measures a viscosity dial reading for each sample. The dial readings may be recorded at various shear rates and/or revolutions per minute, for example 2, 3, 4, 5, 6, or more different shear rates. The well modeling system 138 may have a processor and a memory for storing data and running instructions. To collect the samples, the rheometer 142 can be installed, as illustrated, in line with the hose 128. In other embodiments, the rheometer 142 may be located remotely from the drilling rig 102, and samples may be conveyed to this remote location periodically.

[0013] The rheometer 142 may include, for example, a rotating cylinder that imparts a rotational force on the drilling fluid and measures that force with a torsional bob. Other geometries and measurement techniques may be used to measure the viscosity dial reading for each sample. In the test of each sample, a temperature and pressure are selected (e.g., randomly, progressively) and the rheometer 142 monitors and keeps these selected parameters constant for the sample as a number of shear rates (e.g., the various rotational speeds of the rheometer 142) are tested. The viscosity dial readings for each shear rate are stored by the well modeling system 138, for example, in a computer memory for storing the viscosity dial readings.

[0014] A goal of the modeling is to be able to accurately model a complete picture of the pressure losses in the drilling fluid downhole. This complete picture is known as the equivalent circulating density (ECD) and can be dependent on the movement of the drilling fluid within the borehole. That is, if the drilling fluid were still, the pressure could be modeled and/or calculated more simply using the density of the fluid. Since the drilling fluid constantly flows during the drilling process, however, the rheological properties must be accounted for in the model.

[0015] Rather than taking real time measurements for all temperatures and pressures that may be present in a borehole of a drilling operation, the system 100 periodically takes shear stress dial readings at a combination of temperature and pressure. The combination is based on selection criteria that has a high likelihood of representing a broad range of temperatures and pressures over the course of several measurement periods. For example, the combination may be randomly selected, or may be selected by systematically measuring from a low temperature/pressure, or vice versa. The shear stress dial readings are then ranked according to the relevance of each data set. For example, the shear stress dial readings may be ranked based on how recently they were taken. The system or method then scales a generalized rheology function of temperature and pressure with the time modified weighting. The scaled rheology function is used to model hydraulics for the borehole.

[0016] For example, after a set amount of time (e.g., 45 minutes, 1 hour, or more, or less), a new sample is collected and the rheometer 142 selects a new temperature and pressure to test the shear rates. The well modeling system may select the temperature and pressure randomly from a list and/or range of possible temperatures and pressures (see FIG. 2 and description below). The list and/or range of possible temperatures and pressures may be adjusted by the well modeling system 138 as the borehole 106 changes. For example, as the borehole 106 gets deeper the possible maximum pressures may get higher. Therefore, the maximum pressure in the list and/or range would increase. Furthermore, the list and/or range may be customized to each well based on geological characteristics that are already known about the formations 122. That is, for locations known to have particularly hot formations 122, a range for the temperatures of the borehole being drilled there will include a higher maximum temperature than a range given for a borehole having cooler formations 122.

[0017] From the recorded viscosity dial readings, the well modeling system 138 uses a model (e.g., linear, non-Newtonian, Bingham, power law, Herschel- Bulkley) to scale a generalized rheology response to the range of pressures and temperatures that may be experienced within the borehole. The model may calculate rheological properties for the whole range of pressures and temperatures based on readings from just one combination of pressure and temperature. The model may be improved, however, by additional readings at other temperatures and pressures. For example, a first viscosity dial reading may be measured at the low end of the pressure range, and the low end of the temperature range. This first viscosity dial reading may be used to model the pressure and temperature at the high end of the pressure and temperature ranges. The model may be improved by taking an additional viscosity dial reading, and combining the generalized rheology response from both. The resulting response may be used to calculate the rheological parameters for the fluid at conditions experienced at particular locations downhole. This combination of generalized rheology responses may be done for any number of viscosity dial readings (e.g., 3, 4, 5, 6, 7, 8, or more viscosity dial readings).

[0018] Once the model is complete, the system 100 can change operational parameters to optimize drilling and keep the downhole pressure within a range. In operation, the system 100 manages the rate of the mud pump 124, the rotation of the drill string 112, and the mud density and viscosity. Changing the density and viscosity effects the relationship between movement of the fluid, and the pressure that it exerts on the formation. For example, the system 100 may include functionality to add a viscosifier if the drilling fluid is not viscous enough. If the drilling fluid is too viscous, a thinner may be added. The system 100 may add barite or other materials if the drilling fluid density is too low. Conversely, the system 100 may dilute the fluid with a base fluid or centrifuge out some of the weighting material if the density is too high. These operational changes maintain the pressure, ECD, pore pressure within a production formation, and fracture gradient while maximizing the rate of penetration.

[0019] FIG. 2 is a list 200 of possible temperatures 202 and pressures 204 that a well modeling system (e.g., the well modeling system 138 of FIG. 1) may use to test and record the viscosity dial readings. The range of temperatures 202 and the range of pressures 204 may be customized to a borehole, and may change during the drilling operation as explained above. For example, the range of temperatures 202 may be between -18 degrees Celsius (0 degrees Fahrenheit) to 205 degrees Celsius (400 degrees Fahrenheit). In shallower boreholes, the max temperature may be 100 degrees Celsius (212 degrees Fahrenheit), increasing as the borehole is drilled deeper. The specific values within the range may differ by a set amount, or may be any values between the low end of the high end of the range. The range of pressures may include 0 Pascal (0 psi) to 1.7 x 10 ^L 8 Pascal (25,000 psi), or smaller ranges.

[0020] The well modeling system 138 may select the temperature 202 and pressure 204 independently, or the well modeling system may select the temperature 202 and the pressure 204 from sets 206 of temperature/pressure that are pre-determined combinations of temperature and pressure. In some instances, the well modeling system selects one temperature 202 and pressure 204, records the viscosity dial readings from a rheometer, and then randomly selects a new temperature 202 and pressure 204 after a given time period. The time period may range from a half an hour to several hours. Periodicity may depend on several factors including rate of expected change in the drilling fluid, speed of drilling, drilling equipment, weather, or other considerations. In other instances, the well modeling system may select one temperature 202 to be used for several viscosity dial readings in a row, randomly selecting a new pressure 204 after each given time period. In still further instances, the well modeling system may select one pressure 204 for several viscosity dial readings in a row, randomly selecting a new temperature 202 after each given time period. The well modeling system may also systematically select the temperature 202 and pressure 204. For example, each new temperature 202 and/or pressure 204 is higher than the previous temperature 202 or pressure 204. After several iterations, a broad range of temperatures 202 and pressures 204 will have been selected, and the viscosity dial readings recorded at various shear rates and/or revolutions per minute.

[0021] FIG. 3 is a list 300 of example data sets 302-316 that were recorded by a well modeling system (e.g., the well modeling system 138 of FIG. 1). The list 300 includes the eight most recent sets 302-316 ordered according to when the set was recorded. Each set 302-316 in the list 300 includes viscosity dial readings 318 taken at six different shear rates (e.g., six different rotation speeds of the rheometer). The six shear rates will be common for each of the sets 302-316. In certain instances, the six shear rates will include: 1022, 511, 341, 170, 10.2, and 5.1 reciprocal seconds (1/s). In other instances, more shear rates may be tested, or fewer shear rates may be tested. As may be appreciated, the shear rates may include other values as well for testing the drilling fluid.

[0022] Each of the sets 302-316 is associated with a pressure value 322 and a temperature value 324. These values 322, 324 are selected by the well modeling system as described above. The viscosity dial readings 318 at one pressure value 322 and temperature value 324 may be used to create a generalized rheology response for all temperatures and all pressures using known techniques such as Herschel-Bulkley models.

[0023] To replicate and/or model as close as possible the real-time concurrent rheology tests at a full range of pressure values 322 and temperature values 324, each set 302-316 of viscosity dial readings 318 also includes a weighting value 326. The weighting value 326 for the latest set 302 illustrated in FIG. 3, for example, is 0.50. The well modeling system uses this weighting value 320 in a modeling function to scale a generalized rheology response to pressure and temperature. The weighting value 326 is updated each time a new viscosity dial reading 318 is taken. The older viscosity dial readings 318 are weighted less, and the latest viscosity dial reading is given the highest weighting value 326. The scaling of the weighting values 326 may be based a variety of scaling functions, depending on the expected change in the drilling fluid. For example, the weighting values 326 may decrease according to a linear, parabolic, or exponential weighting function. Or, the eight most recent sets 302-316 may be given custom weighting values 326 based on the knowledge of an operator.

[0024] The generalized rheology response created from the latest set 302 is scaled according to the weighting value 326. That scaled rheology response from the latest set 302 is then combined with the rheology response from the second latest set 304 as it has been scaled by the weighting value 326 (i.e., 0.32). This scaling and combination is completed for each of the sets 302-316 until a shear stress vs. shear rate function for viscosity dial readings is generated for the full range of temperatures and pressures.

[0025] FIG. 4 illustrates a shear stress vs. shear rate function 400 for viscosity dial readings 402 that is generated for the full range of pressures 404 and temperatures 406. In the model 400, it is possible that only a few, or none of the values match the exact values that were measured by the well modeling system. Rather, the model 400 is a combination of all the sets (e.g., the sets 302-316 illustrated in FIG. 3), with the latest set (e.g., the latest set 302 from FIG. 3) having the most highly scaled values. This combination thus increases the accuracy of the model despite changing rheology in the drilling fluid, and, the inherent issues of modeling temperatures and pressures with only one reference point. [0026] Once the model 400 is complete, a substantially real-time ECD can be determined for the length of the borehole. This information may be combined with information about formation downhole to control operating parameters of the drilling operation. For example, if the model 400 shows that a viscosity for conditions at a certain depth is increasing, and that depth has a formation with a low pressure threshold, then the drilling system may lower pressurization from the mud pump to protect the formation. If the viscosity decreases, then the pressure may be re-adjusted back up. Furthermore, the drilling system may also change the rotation speed of the drill string, and/or add additives to the drilling fluid to change the mud density or viscosity. The model 400 may also be updated whenever a new viscosity dial reading is measured.