The Mechanical Stratigraphy of Kerendan Field in Upper Kutai Basin: Implication to Field Development with Massive Carbonate Tight Gas Reservoir

Understanding of field mechanical stratigraphy in terms of formation behavior due to coupled interaction between formation pressure depletion and state of stresses is crucial to achieving successful field development. These provide technical advantages of having a solid foundation for implementation in advanced well construction and completion strategy, especially in light of emerging and challenging plays related to unconventional reservoirs. This paper describes full-field interaction between formation behavior in 4D Geomechanical analysis of Kerendan Field located in Upper Kutai Basin, Central Kalimantan area on gas-condensate production from massive carbonate tight gas reservoir. Integrated 1D/3D/4D geomechanics study workflow result has enabled characterization of each mechanical stratigraphy unit, as follows: The overburden section is comprised of Miocene deltaic clastic succession which is characterized as “soft formation” with low stiffness (Static Young’s Modulus of 0.5 to 1.8 Mpsi) and low - medium rock strength (UCS of 800 to 2000 psi); Reservoir section comprised of Oligocene tight carbonates platform which characterized as “hard formation” with medium stiffness (Static Young’s Modulus of 3.0 to 4.5 Mpsi) and medium rock strength (UCS of 5000 to 6900 psi); Underburden section comprised of Eocene mixed-carbonate clastic succession and Pre-Tertiary metasediments which characterized as “very hard formation” with high stiffness (Static Young’s Modulus of 4.5 to 5.0 Mpsi) and medium rock strength (UCS of 6500 to 7900 psi). The Kerendan field would require implementation of special drilling and stimulation techniques in order to achieve optimum full field development potential owing to its reservoir characteristics. The field’s exhibit a large areal extent and massive tight limestone reservoir with relatively high Young’s Modulus, which is favorable for the utilization of extended reach drill (ERD) / horizontal wells followed with multi-stage acid fracturing stimulation. 3D/4D Geomechanical analysis is essential to assess the drillability and engineering limits of various development scenarios which will be strongly controlled by geomechanical fabric, pre-existing deformation/local discontinuities, prevailing principal stress tensor and stress changes during field production.

Quantitative Prediction of Natural Fractures in Shale Oil Reservoirs

Natural fractures are the key factors controlling the enrichment of shale oil. It is of great significance to clarify the distribution of natural fractures to guide the selection of sweet spots for shale oil. Taking the Qing-1 Member shale oil reservoir in the northern Songliao Basin, China as an example, a new method considering the factors affecting fracture distribution was proposed to quantitatively predict the structural fractures. And the effect of natural fractures on shale oil enrichment was discussed. Firstly, the types and characteristics of fractures in shale oil reservoirs are characterized by using core and outcrop data. Combined with the experimental analysis, the influences of fault, mechanical stratigraphy, mineral composition and content, TOC, and overpressure on fracture intensity were clarified. Then, the number and density of fractures are quantitatively predicted according to the power-law distribution of fault length. Next, geomechanical simulation and fracture prediction were carried out on the model which was established with comprehensive consideration of the influencing factors of fracture distribution. Finally, the fracture distribution is evaluated comprehensively based on above prediction. The prediction results in this work are consistent with the core measurements.

The Mechanics of Initiation and Development of Thrust Ramps

We examine the mechanics of thrust fault initiation and development in sedimentary rocks which accounts for vertical variation in mechanical strength of the rocks. We use numerical mechanical models of mechanically layered rocks to examine thrust ramp nucleation in competent units, and fault propagation upward and downward into weaker units forming folds at both fault tips. We investigate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, fold formation, and fault geometry motivated by the geometry of the Ketobe Knob thrust fault in central Utah. The study incorporates finite element models to examine how mechanical stratigraphy, loading conditions, and fault configurations determine temporal and spatial variation in stress and strain. We model the predicted deformation and stress distributions in four model domains: (1) an intact, mechanically stratified rock sequence, (2) a mechanically stratified section with a range of interlayer frictional strengths, and two faulted models, (3) one with a stress boundary condition, and (4) one with a displacement boundary condition. The models show that a dramatic increase in stress develops in the competent rock layers whereas the stresses are lower in the weaker rocks. The frictional models reveal that the heterogeneous stress variations increase contact frictional strength. Faulted models contain a 20° dipping fault in the most competent unit. The models show an increase in stress in areas above and below fault tips, with extremely high stresses predicted in a ‘back thrust’ location at the lower fault tip. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with the ramp-first kinematic model and development of structures was significantly influenced by the nature of the mechanical stratigraphy.

Mechanical zonation of rock properties and the development of fluid migration pathways: implications for enhanced geothermal systems in sedimentary-hosted geothermal reservoirs

Oil and gas operations in sedimentary basins have revealed the occurrence of significant temperature anomalies at depth, raising the possibility of major geothermal resource potential in the sedimentary sequences. The efficient development of such a resource may require enhancement by hydraulic stimulation. However, effective stimulation relies on an initial assessment of in situ mechanical properties and a model of the rock response. Here, we examine the distribution of mechanical properties (unconfined compressive strength, UCS; ultrasonic velocity-derived Poisson ratio, ν; and, scratch toughness, Ks) along the cored interval of a sedimentary formation with a known low-to-medium temperature geothermal anomaly in the Permian Basin, U.S. Our results reveal the presence of mechanical stratigraphy along the core, demonstrated by the alternation of distinct soft–hard (i.e.,less stiff-to-stiff) mechanical zone couplets composed of: (1) mechanically softer 0.17-m-thick Zone-A and 0.18-m-thick Zone-C with mean values of UCS = 110 MPa, ν = 0.25, Ks = 1.89 MPa·√m; and (2) mechanically harder 0.41-m-thick Zone-B and 0.15-m-thick Zone-D which show mean values of UCS = 166 MPa, ν = 0.22, and Ks = 2.87 MPa·√m. Although X-ray diffraction analyses of the samples suggest that the entire rock matrix is dominated by dolomite, the harder zones show an abundance of quartz cement (> 30%) and relatively lower phyllosilicate mineral content (< 2%) than the softer zones. Further, we observe that the mechanically harder zones have the greatest occurrences and thicknesses of hydrothermal alterations (anhydrite veins and nodules), indicating that the rock had experienced hydrothermal fluid circulation (basinal brines) in the past. We infer that the mechanical stratigraphy most likely influenced the spatial clustering of fractures that facilitated hydrothermal fluid migration in the past, and provides insight that is relevant for the exploitation of geothermal energy resources in sedimentary basins. We suggest that the harder zones or formation intervals with higher ratios of the hard zones relative to soft zones represent viable targets for hydraulic stimulation of a sedimentary-hosted geothermal reservoir, both for the emplacement of new fractures and the linkage of pre-existing fractures to allow efficient fluid circulation. Our findings in this study provide insight that is relevant for understanding the complexity of pre-existing mechanical heterogeneity in sedimentary-hosted geothermal reservoir targets in other places.

Mechanical stratigraphy in Mesozoic rocks of the San Juan Basin: Integration of stratigraphic and structural terms and concepts

We characterize relationships between stratigraphy and natural fractures in outcrops of Mesozoic strata that rim the San Juan Basin in New Mexico and Colorado. These outcrops expose fluvial and shallow-marine siliciclastic deposits and calcareous mudstones deposited in a distal marine setting. We focus primarily on a regionally extensive fracture set formed during the Eocene to minimize localized tectonic effects on fracture development. Where possible, we supplement our observations with wireline log- or laboratory-derived measurements of rock properties. Our goals are twofold: 1) to illustrate how direct integration of data and concepts from stratigraphy and structural geology can lead to better fracture characterization, and 2) to develop thought processes that will stimulate new exploration and development strategies. Genetic beds form one scale of stratification in the outcrops we describe. For example, sandstone beds can be arranged into coarsening and thickening upward successions that are the depositional record of shoreline progradation. In fluvial settings, cm- to dm-scale sandstone beds can also be part of m-scale single-storey channel complexes that, themselves, can be arranged into amalgamated channel complexes 10s of m thick. In these and other settings, it is important to distinguish between beds and features that can be defined via wireline logs because it is the former (cm- to dm-scale) that are usually the primary control the distribution of natural fractures. The extension fractures we describe are typically bed-bound, with bedding being defined by lithology contrasts and the associated changes in elastic properties. Fracture spacing distributions are typically lognormal with average spacing being less than bed thickness. Although mechanical bedding and depositional bedding are commonly the same, diagenesis can cut across bed boundaries and complicate this relationship, especially where lithologic contrasts are small. Deposits from similar depositional environments which undergo different diagenetic histories can have substantially different mechanical properties and therefore deform differently in response to similar imposed stresses.

Mechanical stratigraphy of Mississippian strata using machine learning and seismic-based reservoir characterization and modeling, Anadarko Basin, Oklahoma

The STACK (Sooner Trend in the Anadarko [Basin] in Canadian and Kingfisher counties) play primarily produces oil and gas from Mississippian strata. The interval consists of interbedded argillaceous mudstones and calcareous siltstones. Such contrast in rock composition is linked directly to the mechanical stratigraphy of the strata. Brittle (calcareous siltstones) and ductile beds (argillaceous mudstones) are related to the sequence-stratigraphic framework at different scales. We use seismic and well log data to estimate and map geomechanical properties distribution and interpret the mechanical stratigraphy of rocks within the Mississippian strata. First, we defined the parasequences that form the main reservoir zones of the Meramecian-Mississippian strata. Once we established the stratigraphic framework, we estimated and compared rock brittleness index using two independent laboratory-based measurements from the core. The first method MIDBI (MIneralogical Derived Brittleness Index), uses mineralogical composition inverted from FTIR (Fourier Transform InfraRed spectroscopy) analyses, whereas the second method MEDBI (MEchanical Derived Brittleness Index), involves measurements of compressional and shear velocities from core plugs.We use the data from core-plug velocity measurements and well logs and an Artificial Neural Network (ANN) approach to establish relationships between geomechanical properties, well logs, and acoustic impedance values. We then applied these relationships to generate 3-D geomechanical models constrained to seismic volumes. The resulting grid distributions illustrate the stratigraphic variability of the properties at the parasequence scale. Overall, brittle strata decrease in thickness and abundance basinward as the frequency of interbedded brittle and ductile zones increases and gradually transitions into thin calcite-cemented siltstones and clay-rich mudstones. Analysis of the production performance of selected horizontal wells drilled within the Mississippian strata shows that the proportion of brittle and ductile rocks along the well path drilled and the drilled area vertical stacking pattern play a significant role in the hydrocarbon production for these Mississippian units.