The Bowland Basin in Northern England contains a thick shale gas section (>5,000 ft) estimated to hold over 1300 TCF of total original gas in place of shale gas resource. In 2017, Cuadrilla Resources drilled a S-shaped pilot well, Preston New Road-1 (PNR-1), located in Lancashire, NW England. The plan was to drill, core, and log the Bowland Shale sequence with the primary objective to select the optimum landing depth for a subsequent side-tracked horizontal section (PNR-1z) of up to 3,280 ft length to be completed for multi-stage hydraulic fracturing. Another multi-stage horizontal well, PNR2, was also planned to be drilled afterward targeting a different stratigraphic horizon. Three vertical wells (PH-1, GH-1 and BS-1) were previously drilled in the Bowland Basin to a depth of 8,860-10,500 ft. Delays were encountered in the drilling of these wells due to multiple borehole stability problems. Specifically, in GH-1, the well required a side-track to reach the target depth. With the plan to drill four horizontal wells at Preston New Road, the first horizontal wells ever to be drilling in the Bowland shale, a rigorous geomechanical study was required to provide valuable insights for optimisation of the drilling programme.
A pre-drill geomechanical model was developed for the PNR-1 pilot well using advanced interpretation of available data and the gained experiences from the offset wells. A comprehensive pore pressure interpretation showed that Bowland shale is significantly over-pressured (0.69 psi/ft). The model was backed up by the observed splintery cuttings and gas shows in offset wells. It was concluded that this abnormal pore pressure combined with a tectonic strike-slip stress regime (with large horizontal stress anisotropy) and intrinsic anisotropic shale properties were the primary causative factors for drilling incidents. As a result of this study, the PNR-1 was successfully drilled and completed with minimal borehole stability problems despite the presence of narrow operating mud weight window in several stratigraphic intervals. The data acquisition program conducted included 114m of core from Upper and Lower Bowland shales, with the required logs for updating the geomechanical model. A comprehensive rock mechanics testing program was designed and conducted which resulted in better characterizing the anisotropic elastic properties and strength parameters of the Bowland Shale. This information was used to update the geomechanical model and aid the optimum landing decision depth of 2,180m for PNR-1z. A successful XLOT prior to drilling the 6" lateral section provided valuable data for further calibration of the stress model. The updated model was then used to develop safe operating mud weight window for PNR-1z, which helped drilling of the horizontal section to the TD at 11,233 ft MD (7,457 ft TVD) with no notable drilling problems.
This paper presents a summary of the geomechanical work performed for successful drilling and hydraulic fracturing operations in the Preston New Road exploration site and the outcomes and achievements.
Hjeij, Dawood (Division of Sustainable Development, College of Science and Engineering, Hamad Bin Khalifa University) | Abushaikha, Ahmad (Division of Sustainable Development, College of Science and Engineering, Hamad Bin Khalifa University)
This paper investigates the performance of the mimetic finite difference (MFD) discretization scheme for modelling fluid flow in anisotropic porous media. We apply numerical benchmark studies on the MFD scheme to measure its accuracy when the horizontal permeability is much larger than the vertical one in a diagonal permeability tensor. We also run full-field simulations to investigate the modelling capability of this method and compare it to other advanced discretization schemes.
Quasi K-orthogonal grid generation is presented, to improve grid quality and method stability with respect to flux approximation in the presence of strongly anisotropic full-tensor permeability fields.K-orthogonal grid generation is only possible for low anisotropy ratios. Quasi K-orthogonal grid generation involves satisfying the K-orthogonal condition approximately, resulting in grids that place less demand on an approximation with respect to stability conditions, and therefore improve grid quality with respect to flux approximation in the presence of anisotropic permeability fields. The method employed enables Delaunay grid generation principles to be employed in a locally transformed system according to local permeability tensor variation. The resulting method has great flexibility for handling complex geometries and can handle jumps in permeability tensor principal axes orientation and jumps in coefficients and details will be presented. Results are presented that demonstrate the benefit of a quasi K-orthogonal grid. Highly challenging cases involving strong full-tensor permeability fields where control-volume distributed multi-point flux approximation (CVD-MPFA) schemes exceed their stability limits and yield solutions with spurious oscillations when using conventional grids, are solved using the new grid generation method. CVD-MPFA schemes are still required as the grids are only approximately K-orthogonal in such cases, however the schemes retain a discrete maximum principle on the new quasi-K-orthogonal grids and yield well resolved solutions that are free of spurious oscillations. While the two-point flux approximation (TPFA) requires strict K-orthogonality, results using both CVD-MPFA and TPFA will be presented. New Quasi K-orthogonal grid generation methods are presented that satisfy the K-orthogonal condition approximately, resulting in practical grids that restore a discrete maximum principle (stability) for the CVD-MPFA schemes when applied to cases involving general full-tensor permeability fields. Results are presented for a variety of test cases that confirm the validity of the grids.
Nandi, Anindya (Schlumberger) | Sarkar, Subhadeep (Schlumberger) | Chatterjee, Chandreyi (Schlumberger) | Das, Sourav (Schlumberger) | Pattanaik, Sambit (Schlumberger) | Majumder, Chandan (Schlumberger) | Haldia, Bhopal Kumar (Oil & Natural Gas Corporation Ltd.) | Chaturvedi, Praveen Chandra (Oil & Natural Gas Corporation Ltd.) | Srivastava, Siddharth (Oil & Natural Gas Corporation Ltd.) | Verma, Malay (Oil & Natural Gas Corporation Ltd.) | Sarkar, Sutanu (Oil & Natural Gas Corporation Ltd.)
Owing to the depleting reserves in the conventional reservoirs over the last few years, unconventional reservoirs have gained significant importance in the exploration of oil and gas. Basement rocks, though non-sedimentary in origin, is looked upto as one of the important unconventional reservoirs. Deccan volcanics in Kutch-Saurashtra is one such example from India. This study shows and validates a methodology of how acoustic log data can be integrated with borehole images to understand reservoir properties that governs flow. It has been noticed that presence of open fractures is not the single biggest driver contributing to production. Insitu stress plays a critical role in guiding fracture mobility. To understand and determine which fractures would contribute to flow, a geomechanical study of performing the fracture stability analysis has been carried out. This generates a Mohr circle plot that defines the Mohr-Colomb shear failure criteria using the stress and critical fracture angles. Combining these three-way approaches of acoustic, image log and geomechanics, a workflow has been established for this field to identify fractures and quantify the permeable zones. This workflow has been used for two nearby wells in this field and subsequent result emphasises the utility of this method to find out sweet spots of fluid flow in fractured basement.
Liu, Jinpeng (Data Processing Company, Geophysical-COSL) | Zhong, Mingrui (Data Processing Company, Geophysical-COSL) | Fang, Zhongyu (Data Processing Company, Geophysical-COSL) | Dan, Zhiwei (Data Processing Company, Geophysical-COSL) | Sun, Leiming (Data Processing Company, Geophysical-COSL)
With the deepening of exploration and development, exploration in the south China sea is faced with increasingly complex geological targets, including complex fault blocks, lithological targets, middle and deep strata, small scales and more subtle and complex exploration targets. At present, the internationally recognized best seismic solution is "two wide and one high" acquisition and processing, namely wide azimuth, broadband, high-density field observation system and targeted processing. In the aspect of wide azimuth acquisition and processing, domestic land acquisition has also developed greatly, and in the past few years, there has also been a heated debate on the advantages and disadvantages of wide and narrow azimuth acquisition in complex areas. However, in the aspect of offshore acquisition, wide-azimuth acquisition is rarely carried out. The main reason is that the construction cost and difficulty are higher. With the primary 3d coverage of some mature areas on the sea, it is still unable to meet the exploration needs. In the past, the seismic observation system design was mostly based on the seismic acquisition and survey lines in accordance with the direction of vertical structure strike, so as to facilitate the accurate imaging of the main structure or the construction along the long axis of the work area according to the cost of offshore acquisition. However, in fact, the fault strike in the tectonic development area is complex and changeable, and there is no uniform rule. Some small faults that control the trap are completely perpendicular to the large structural strike, so the old 3d will lead to some poor fault imaging. According to the practical test data analysis, found that the different line direction observed data imaging effect is different, therefore some recently in the south China sea area for the secondary multi-dimensional three-dimensional attempt at sea to achieve multiple acquisition costs are relatively low, and the construction is convenient, but if you want to achieve benefit maximization, must consider the joint use of composite materials. Compared with the factors that need to be considered in collection and processing, it is much more complex, and the development of processing is relatively lagging behind. The main anisotropy is not fully considered, and relatively simple superposition is often difficult to reflect the effect of multi-direction.
The existence of elastic anisotropy in the reservoir is obtained through the equivalent media theory. An isotropic elastic theory fairly explains the reservoir modeling or characterization but its not well explain anisotropic characteristic fairly for reservoir characterization which is extremely challenging without considering a self-consistent theory of effective equivalent media theory. In this research, equivalent media theory has been explained and implemented on a producing well-log data with consistent Vp, Vs, density and other parameters. Instead of using Voigt averaging, equivalent media theory used to estimate the effective stiffness parameters and compare with Thomsen's parameters and finally used effective anisotropy parameters and compare with gamma log. Result shows the effectiveness of equivalent media theory for future application for developing reservoir modeling and characterization.
Understanding in-situ stress orientations and magnitudes is critical in building a geomechanical model which helps in planning and execution of a development and production programme for any hydrocarbon reservoirs. The azimuthal anisotropy analysis from cross-dipole acoustic data is commonly used to derive the direction of maximum-horizontal stress. However, the interpretation of the stress orientation is challenging in inclined wells where anisotropy may also be influenced by the relative angle of bedding plane to the bore hole. Integration of the data becomes of paramount importance to correctly interpret the stress distribution.
Cross dipole wireline acoustic, 3D resistivity and 6 arm calliper data were acquired in a deviated well, offshore, Malaysia. Acoustic data was processed for azimuthal anisotropy, 3D-resistivity data was processed for formation dip, azimuth, horizontal and vertical resistivity and 6 arm calliper data was used to generate borehole shape. Acoustic analysis provided the difference in fast and slow shear wave velocities and the azimuth of fast shear. The resistivity anisotropy, dip and azimuth and bore hole shape information was incorporated to interpret effect of the dipping bed in the scheme of relating acoustic anisotropy to the formation stress.
Meaningful difference in the fast and slow shear velocities (in two orthogonal direction) is observed in this well. The fast shear wave azimuth of NW-SE is consistent with the regional trend. However, the presence of laminated shale interval in the inclined bore hole imparts uncertainty in relating the anisotropy to the stress field. The formation dip and azimuth obtained from the resistivity anisotropy provided the framework of the interpretation by identifying the intervals with higher relative dip and the associated anisotropy perceived by it. Bore hole ovalization also provides the necessary input to the interpretation scheme which is supported by the existing field wide geomechanical model.
Integrating all datasets resolved potentially ambiguous interpretation of the source of azimuthal acoustic anisotropy. This approach determines the cause of the anisotropy (unbalanced stress in formation vs. dipping beds and shale transverse anisotropy). The result provides valuable information to refine the existing geomechanical model which can be used in future well placement and planning, optimum mud weight design, and constraining water injection operating limit during the life of the field.
Guan, Lijun (CNOOC Shenzhen) | Wang, Xiannan (CNOOC Shenzhen) | Xiao, Dong (CNOOC Shenzhen) | Shim, Yen Han (Schlumberger) | Maggs, David (Schlumberger) | Maeso, Carlos (Schlumberger) | Legendre, Fabienne (Schlumberger) | Leech, Richard (Schlumberger) | Qu, Chang Wei (Schlumberger)
During field testing of a logging-while-drilling (LWD) laterolog resistivity and imaging tool, formation resistivity differences were observed between the new laterolog and standard propagation resistivity. This paper compares the resistivity measurement acquired in the same borehole using different tools in both sand and shale formations. In addition, the high-resolution images acquired by the new tool are used for a detailed geolgical analysis of the sequence.
The high-resolution images acquired by the tool are used to determine the sedimentary environments in this complex fan delta sequence. A wide range of facies types can be identified on the images and correlated to available core with detailed examples shown of the key reservoir facies (distibutary channels and mouth bars). The images also provide valuable structural, depositional trend and insitu stress information for this well.
The laterolog resistivities were higher in the shales and lower in the sands than the propagation resistivity values. The data was acquired while drilling in a water-based mud, sub-vertical exploration well in the South China Sea. While the main objective of the data acquisition in the siliciclastic formations was high-definition resistivity borehole images for detailed geological description, the radial laterolog resistivity response was also of interest. An advanced wireline multi-frequency dielectric measurement was also acquired, and its response was used for comparison and validation.
In this paper, we associate the differences in resistivity response for varying formation properties to the tool physics, vertical resolution, depth of investigation, and time after bit between the measurements. In the sands, a resistivity inversion was applied to correct the logs for invasion effects and forward modeling used to resolve the resolution differences. The inverted formation resistivity from the LWD laterolog matches the deeper reading LWD propagation resistivity. The shale response was initially found to be more difficult to explain. It is commonly and historically accepted that due to resistivity anisotropy laterolog reads higher than propagation resistivity in low angle wells with laminated formations. Advanced forward modeling was used to investigate the laminations observed on the high-definition images and high-resolution laterolog resistivity curves. Although a model could be created to match both sets of resistivity measurements, the level of anisotropy required was considerably higher than expected, and supplementary information was required to validate the model. The wireline multi-frequency dielectric measurements provided the additional information required to confirm the anisotropy contrast observed by the resistivity modeling and confirm the LWD tool responses.
This paper will compare the tool responses, and to determine the correct sand and shale resistivity. It will show how by combining different measurements, additional insight can be obtained into the nature of the formation and its properties.
Kumar, Rajeev (Schlumberger) | Zacharia, Joseph (Schlumberger) | Guo Yu, Dai (Schlumberger) | Singh, Amit Kumar (Schlumberger) | Talreja, Rahul (Schlumberger) | Bandyopadhyay, Atanu (Schlumberger) | Subbiah, Surej Kumar (Schlumberger)
The unconventional reservoirs have emerged as major hydrocarbon prospects and optimum yield from these reservoirs is dependent on two key aspects, viz. well design and hydrofracturing wherein rock mechanics inputs play key role. The Sonic Measurements at borehole condition are used to compute the rock mechanical properties like Stress profile, Young's Modulus and Poisson's Ratio. Often, these are influenced by the anisotropy of layers and variations in well deviation for same formations. In one of the fields under review, the sonic compressional slowness varied from 8us/ft. to 20us/ft. at the target depth in shale layer in different wells drilled with varying deviation through same formations. This affected the values of stress profile, Young's Modulus and Poisson's Ratio resulting in inaccurate hydro-fracture design. At higher well deviation, breakouts were frequently observed and could not be explained on the basis of compressional slowness as it suggested faster and more competent formation. Current paper showcases case studies where hole condition improved in new wells with better hydro fracturing jobs considering effect of anisotropy in Geomechanics workflow. Sonic logs in deviated wells across shale layer were verticalized using estimated Thomson parameters considering different well path through same layer and core test results. Vertical and horizontal Young's Modulus and Poisson's Ratio were estimated for shale layers with better accuracy. The horizontal tectonic strain was constrained using radial profiles of the three shear moduli obtained from the Stoneley and cross-dipole sonic logs at depth intervals where stress induced anisotropy can be observed in permeable sandstone layer. A rock mechanics model was prepared by history matching borehole failures, drilling events and hydro-frac results in vertical and horizontal wells using updated rock properties. Geomechanical model with corrected sonic data helped to explain the breakouts in shale layer at 60deg-85deg well deviation where the original sonic basic data suggested faster and more competent formation with slight variation in stress profile among shale-sand layer. Considering shear failure, the mud weight to maintain good hole conditions at 80deg should be 0.6ppg-0.8ppg higher than that being used in offset vertical wells. Estimated closure pressure and breakdown pressure showed good match with frac results in deviated wells using new workflow. There was difference of .03psi/ft-0.07psi/ft. in shale layers using this new workflow which helped to explain frac height and containment during pressure history match. This paper elucidates the methodology that provides a reliable and accurate rock mechanics characterization to be used for well engineering applications. The study facilitates in safely and successfully drilling wells with lesser drilling issues and optimized frac stages.
Probe permeameter (also known as Mini-permeameter) has been widely used in many field and laboratory applications where in-situ measurements and spatial distributions of permeability are needed. Mini-permeameter measurements have become popular techniques for collecting localized permeability measurements in both laboratory and field applications. It is designed to obtain fast, cheap, intensive and non-destructive permeability measurements and to describe the spatial arrangement of permeability.
In this work, the effect of vertical and horizontal anisotropy on the probe permeameter measurements was investigated. A numerical simulation model for the rectangular system representing a porous rock sample was built based on finite difference discretization of steady-state flow of an incompressible single-phase fluid in a three-dimensional (3D) system. The seal and tip of the probe permeameter are represented by no-flow boundary and constant pressure boundary, respectively. All the sides of the sample are represented by a constant-pressure boundary in which the sample is exposed to the atmosphere. The investigation was conducted by examining different permeability anisotropy ratios. These include fully isotropic sample, different vertical anisotropy ratios in a horizontally-isotropic sample, different vertical anisotropy ratios at different horizontal anisotropy ratios. All these investigations are performed at a constant probe injection pressure of 50 psig at the injection tip.
The results obtained showed the clear effect of anisotropy on the probe permeameter measurements and were expressed in dimensionless parameters. These dimensionless parameters include the ratio between the flow rate measurements at different directions for different vertical and horizontal anisotropy ratios. They also include the dimensionless pressure drop for the pressure drop measurements at different directions. From the plots of these dimensionless parameters, the permeability at different directions can be evaluated from a few steady-state flow rate and pressure drop measurements at different directions on a rectangular core sample. Therefore, a practical procedure for evaluating permeability anisotropy from probe permeameter measurements is also described.