Paradigm Shift in Drilling to Completion in Unconventional Reservoir, Eastern Onshore, India

Permo-Triassic formations in Mandapetta field from eastern onshore, India possesses historical drilling challenges in terms of wellbore instability, non-productive time and poor hole condition in deep higher stressed formations. Lack of acquiring reliable log data and problems in recovering good quality cores present difficulties in proper formation evaluation and zone selection for testing. Historical well test results in target K-Formation has been not encouraging despite good gas shows during drilling. Estimated formation pressure gradient ranges 1.45sg-1.52sg. Layered shale with coal and tight sandstone in same open hole section pose risks of mud losses and poor cement job. Present study highlights the workflow adopted to improve drilling and completion in open hole section of more than 1000 m with varying lithology being drilled successfully. Advanced 3D anisotropic acoustic measurements acquired are used to estimate anisotropic elastic properties (vertical and horizontal Young's modulus and Poisson's ratio) in the overlying shales. Horizontal tectonics has been determined across stress induced anisotropic layers. This approach provides better understanding of formations and stress distribution. Thomsen Gamma values range 0.1 to 0.4 in shale layers of overburden formations. In order to minimize uncertainty in 8.5inch section while drilling, advanced logs were acquired in 12.25inch hole section to estimate tectonics at well location while constraining ratio of horizontal to vertical Young's modulus and Poisson Ratio in shale layers based on Thomsen Gamma and clay volume. Analysis suggested typical VTI anisotropy of 15%-20% in shale layers. Inverted direct horizontal strain parameters at well location suggested the ratio of maximum to minimum horizontal stress to vary 1.15-1.23. Mud weight used while drilling 8.5inch section ranged 1.49sg1.52sg against the recommended mud weight of 1.50sg-1.52sg while pumping sealing agents to prevent mud losses in coal layers. Flow rate was maintained on lower values to minimize ECD values. Hole condition improved significantly with no issues in logging. Post-drill anisotropic rock mechanics model suggested good quality sandstone in target source formation with usual conventional reservoir in shallower formation. Zone was selected based on permeability, breakdown and completion quality for perforations. Analysis of high-quality sonic slowness helped to identify possible gas reservoir in laminated source rock. There was stress contrast of 2000psi-2500psi among reservoir layers and shale stress barriers. Implemented workflow and successful execution helped to drill the well 5 days earlier than plan with no major drilling incidents. Successful core recovery for Shale Gas evaluation was also possible due to better wellbore quality. Initial testing of K-Formation produced gas with significant improved flow rate by 150% without any stimulation for the 1st time in the history of the field.

An innovative method to simulate stress-induced velocity changes in anisotropic rock

This contribution proposes a numerical microstructural modeling approach to investigate stress-induced seismic velocity changes on anisotropic rocks. By introducing pre-existing cracks with preferential orientations in bonded-particle assemblies, the transverse isotropic structure of the Whitby Mudstone is simulated. Using power-law distributed aperture and calibrated micro-properties, we successfully reproduce stress-dependent velocity changes on Whitby Mudstones with different anisotropic angles in relation to the applied loads. The proposed model also duplicates the directional dependence of wave speed with respect to the bedding plane as expected theoretically. The numerical models show that velocity increase results from the closure of pre-existing cracks due to load increase. Direct relations are established between velocity changes and opened crack density (or crack closure), which displays a similar tendency compared with theoretical predictions. This relation can be used to quantify the micromechanisms behind the velocity changes. The proposed model provides the ability to directly examine the micro-processes underlying velocity changes.

Effect of Fluids on the Elastic Properties of 3D-Printed Anisotropic Rock Models

Laboratory physical models play an important role in understanding rock properties and wave propagation, both theoretically and at the field scale. In some cases, 3D-printing technology can be adopted to construct complex rock models faster, more inexpensively, and with more specific features than previous model-building techniques. In this study, we use 3D-printed rock models to assist in understanding the effects of various fluids (air, water, engine oil, crude oil, and glycerol) on the models’ elastic properties. We first used a 3D-printed, 1-in. cube-shaped layered model. This model was created with a 6% primary porosity and a bulk density of 0.98 g/cc with VTI anisotropy. We next employed a similar cube but with horizontal inclusions embedded in the layered background, which contributed to its total 24% porosity (including primary porosity). For air to liquid saturation, P-velocities increased for all liquids in both models, with the highest increase being with glycerol (57%) and an approximately 45% increase for other fluids in the inclusion model. For the inclusion model (dry and saturated), we observed a greater difference between two orthogonally polarized S-wave velocities (Vs1 and Vs2) than between two P-wave velocities (VP0 and VP90). We attribute this to the S2-wave (polarized normal to both the layering and the plane of horizontal inclusions), which appears more sensitive to horizontal inclusions than the P-wave. For the inclusion model, Thomsen’s P-wave anisotropic parameter (ɛ) decreased from 26% for the air case to 4% for the water-saturated cube and to 1% for glycerol saturation. The small difference between the bulk modulus of the frame and the pore fluid significantly reduces the velocity anisotropy of the medium, making it almost isotropic. We compared our experimental results with theory and found that predictions using Schoenberg’s linear slip theory combined with Gassmann’s anisotropic equation were closer to actual measurements than Hudson’s isotropic calculations. This work provides insights into the usefulness of 3D-printed models to understand elastic rock properties and wave propagation under various fluid saturations.

A Statistical Damage Constitutive Model of Anisotropic Rock: Development and Validation

The damage constitutive model is of great significance to research the stress-strain relationship and damage evolution of rock under loading in engineering. In order to investigate the effect of anisotropic characteristic on the stress-strain relationship and damage evolution, a statistical damage constitutive model of anisotropic rock under true triaxial condition was developed. In this study, the plane which existed perpendicular to the coordinate axis was extracted from representative volume element (RVE) of rock. The extracted plane was assumed to be composed of abundant mesoscopic elements whose failure strength satisfied the Weibull distribution. According to the number of failure elements on the plane in each direction under loading, the anisotropic damage variable was established based on the proposed concept of areal damage. A statistical damage constitutive model of anisotropic rock was developed by using strain equivalent hypothesis and generalized Hooke constitutive model. Subsequently, the parameters in the anisotropic damage constitutive model were determined by the method of total differential. Thus, the damage evolution of anisotropic rock under various stress conditions can be conveniently evaluated by the anisotropic damage model. The model was validated based on the tests of rocks under the stress conditions of conventional triaxial and true triaxial, respectively. Moreover, for the purpose of studying the influence of parameters on the model, sensitivity analyses of mechanical parameters and model parameters were carried out. The results of statistical damage constitutive clearly demonstrate the stress-strain and damage evolution of anisotropic rock under various stress conditions.

Using Continuum-Discontinuum Element Method to Model the Foliation-Affected Fracturing in Rock Brazilian Test

In this study, the continuum-discontinuum element method (CDEM) was used to investigate the tensile fracture mechanism of rock materials. An isotropic rock disk model and models considering different foliation inclinations were established, and three schemes were used to simulate the rock fracturing in Brazilian test. Then, the influences of the rock matrix and foliation strengths on anisotropy rock fracturing were investigated. Furtherly, simulation results were verified, and the rock fracture mechanisms were discussed. The results show that the rock fracturing in Brazilian test can be accurately simulated by CDEM, which is in accordance with the experimental results. For isotropic and horizontal foliation rock, the stress concentration in loading positions causes a local fracture of rock sample, and application of a local strengthening scheme can simulate the integral tension fracture of sample middle. As the foliation angle varies from 15° to 45°, the rock fracturing is affected by the stress concentration and foliation distribution. In splitting simulation, a strengthening scheme should be adopted to overcome this influence. As a result, the rock sample generates the sliding and compression-shear fracture. As the foliation angle changes from 45° to 75°, the foliation, rather than the matrix, dominates the fracture behavior of rock sample. For vertical foliations’ rock, as the middle foliation thickness is appropriately broadened, the simulation results are reasonable. In general, the tensile strength of anisotropic rock entirely decreases with an increase of foliation angle, and the effect of foliation strength on the tensile strength rock sample is larger than that of the rock matrix.

RV/RH ANISOTROPY IN UNCONVENTIONAL FORMATIONS: RESOLVING THE RIDDLE OF RESISTIVITY

The problem of capricious log response is one that has persistently troubled formation evaluation experts since the Schlumberger brothers ran their first log in Pechelbronn, France. Since the advent of 6FF40 induction logs in the 1950’s, subtle differences have been noted between laterolog and induction response. As field resistivity measurements have evolved to array induction and array laterolog tools, resultant resistivity variability has increased. (Gianzero, 1999) This paper examines how the resistivity discrepancies between laterolog and induction response in an electrically anisotropic rock can greatly affect calculated water saturations (Sw), and ultimately oil in place. Further, several possible solutions are posited to resolve the riddle of resistivity. The root cause of the differences between the two measurement techniques is in how each tool measures the vertical resistivity (Rv) and horizontal resistivity (Rh) in addition to dielectric effects. In isotropic formations, the difference between Rv and Rh is miniscule. However most organic shales and many laminated low porosity formations are anisotropic. (Klein et al., 1997) In anisotropic formations, the ratio of Rv/Rh is not constant over the possible range of resistivities. This ratio has been observed to be as high as 5 at less than 1 ohmm of Rh, and approaches unity at infinite resistivity. Due to the high Rv/Rh ratio, at low resistivities, differences between laterolog response (Rh + fraction of Rv) and induction response (Rh) has a dramatic impact on resultant water saturation values. Laterolog array measurements exhibit a systematically higher resistivity than array induction measurements in the same formation. Variances in Sw as high as 30% has been observed. Since most North American unconventional fields have a mix of historical laterolog and induction data from different eras, it is imperative to address this apparent contradiction in values. Further confounding the issue, the mud salinity required to run both tools at peak performance is nearly mutually exclusive. This complicates efforts to resolve the conundrum because the tools cannot be run simultaneously. The closest measurements on the same rock come from sidetracked wells where one has a laterolog and the other induction. The next best possible measurement is the tri-axial resistivity which can be used to model the Rv and Rh. The issue with tri-axial tools, is that the laterolog apparent resistivity does not conform to either end member of the Rv or Rh. Since the detailed field measurements have been lost to time and only the measured resistivities are available in public LAS data sets, several practical solutions have been devised by the authors to untangle this mess. First, sets of proximal wells (<1000 ft apart) with either tool were depth-shifted and oriented for analysis. Wells with tri-axial resistivity modeled Rv and Rh supplemented the data set. Once the data was collected, the authors utilized simple x-y regression, multilinear regression, artificial neural net, and random forest regression to predict true Rh. The results of each predictor algorithm is discussed, as the optimal solution is situationally dependent.