Weak Sandstone Research Articles

Optimizing Perforation Phasing through Nearest Neighbor Lookup and Delaunay Triangulation

Summary Sand production is a common occurrence in reservoirs with weak sandstone undergoing pressure depletion during hydrocarbon extraction. Among available techniques, the adoption of the cased, cemented, and perforated well completions (CCPs) stands out for its inherent safety measures and cost-effectiveness compared to alternative methods. This approach is anticipated to offer substantial potential for bolstering hydrocarbon yields when juxtaposed with the openhole completion methodology, positioning it as a preferable well completion strategy for such reservoirs. In the design of perforation arrangements, the determination of shots per linear foot (SPF, spf) is typically contingent upon the targeted hydrocarbon production capacity. Thus, fine-tuning the phase angle becomes imperative to forestall overlap within the damaged zones surrounding neighboring perforations, thereby mitigating the interaction between them—a pivotal consideration in the design optimization process. In light of the absence of constraints and the relative ease of implementing a single helical perforation pattern (SHPP) over other intricate designs, this research is centered on ascertaining the minimal separation between adjacent perforations, termed the minimum perforation-to-perforation spacing (PPSm), across various phase angles within this pattern. This objective is pursued through a swift exploration of nearest neighbors (NN) facilitated by a spatial-partitioning data structure, specifically a k-dimensional tree (KDTree), aimed at probing the optimal phase angles for customary values of SPF, predicated on a well diameter of 8½ in. Achieving a uniform distribution of the perforations has been realized through the application of mathematical and statistical principles, utilizing a parameter known as the equilateral likeness score (ELS). Employing the Delaunay triangulation, the arrangement of the perforations has been meticulously configured to minimize this score, thereby signifying the most even distribution of them. The optimal phase angles have been discerned from two distinct vantage points—maximizing the spacing between adjacent perforations and maximizing the feasible angle values for the phasing. A comparative analysis between the outcomes derived from the proposed theory and those from prior theories—including the isosceles obtuse triangle pattern (IOTP)—reveals the likelihood of notable disparities in the calculated optimal phase angles. Particularly within the realm of the two-wrapped-based methodologies, variations in the magnitude of these angles can be substantial, extending up to approximately 80° for certain shot densities. Notably, the optimal phase angles of 97° and 77° have been estimated for two prevalent shot densities of 6 spf and 12 spf, respectively, contrasting with the values of 99° and 143° previously obtained using IOTP. This substantial difference in phase angles observed for the shot density of 12 spf has been minimized to approximately 1° through refinement of the previous method, achieved by removing its constraint on the number of wraps.

The Coefficient of Earth Pressure at Rest K0 of Sands up to Very High Stresses

The mechanical behaviour of soils subjected to any stress path in which deviatoric stresses are present is heavily characterised by non-linearity, irreversibility and is strongly dependent on the initial state of stress. The latter, for the majority of geotechnical applications, is normally determined by the at-rest earth pressure coefficient K0, even though this state is valid, strictly speaking, for axisymmetric conditions and for zero-lateral deformations only. Many expressions are available in the literature for the determination of this coefficient for cohesive and granular materials both for normal consolidated and over-consolidated conditions. These relations are available for low to medium stress levels. Results of an extensive experimental investigation on two sands of different mineralogy up to very high stress (120 MPa) are reported in the paper. For reach very high vertical stresses, a special oedometer has been realised. In the loading phase (normal consolidated sands), the coefficient K0n depends on the stress level. It passes from values of about 0.8 to values of about 0.45 in the range of effective vertical stress σ′v = 0.5–4 MPa. Subsequently, K0n is about constant and varies between 0.45 to 0.55 up to very high vertical effective stresses (120 MPa). For the sands employed in the tests, Jaki’s relation did not lead to reliable results at relatively low pressures, while at high pressures, the same relationship seems to lead to reliable predictions if it refers to the constant volume angle of shear strength. For the over-consolidated sands, K0C strongly depends on the OCR, and for very high values of OCR, K0C could be greater than Rankine’s passive coefficient of earth pressure, Kp. This result is due to the very locked structure of the sands caused by the grain crushing, with intergranular contact of sutured and sigmoidal, concavo-convex and inter-penetrating type, that confer to the sand a sort of apparent cohesion and make it similar to weak sandstone.

Effects of reservoir fluids on sand packs consolidated by furan and epoxy resins: Static and dynamic states

One of the most effective methods for sand control is the chemical consolidation of sandstone structures. In this paper, the impacts of crude oil and brine in the static state and the impact of the flow rates of the fluids in the dynamic state have been assessed at the reservoir conditions. The analyses in this research were Young's modulus, compressive strength, porosity, and permeability which were done on core samples after and before fluid contact. Samples made with two different resins showed good resistance to crude oil in both states. No considerable change was seen in the analyses even at high crude oil injection rates in the dynamic state. Conversely, brine caused a noticeable change in the analyses in both states. In the presence of brine at the static state, Young's modulus and compressive strength respectively decreased by 37.5% and 34.5% for epoxy cores, whereas these parameters respectively reduced by 30% and 41% for furan cores. In brine presence at the dynamic state, compressive strength reduction was 10.28 MPa for furan and 6.28 MPa for epoxy samples and their compressive strength reached 16.75 MPa and 26.54 MPa respectively which are higher than the critical point to be known as weak sandstone core. Moreover, Young's modulus decrease values for furan and epoxy samples were respectively 0.37 GPa and 0.44 GPa. Therefore, brine had a more destructive effect on the mechanical characteristics of samples in the static state than the dynamic one for two resins. In addition, brine injection increased permeability by about 13.6% for furan and 34.8% for epoxy. Also, porosity raised by about 21.8% for furan, and 19% for epoxy by brine injection. The results showed that the chemical sand consolidation weakens in the face of brine production along with crude oil which can lead to increasing cost of oil production and treating wellbore again.

Influence of Bedding Strength and Angle on Fracture Characteristics of Sandstone under Three-Point Bending Conditions

The fracture characteristics of bedded sandstone determine the stability and safety of in situ coal gasification technology. Four semicircular stratified sandstone specimens with different strengths (0.3, 0.6, 1.0, and 1.5 times that of rock matrix) and seven different bedding angles (θ = 0°, 15°, 30°, 45°, 60°, 75°, and 90°) were numerically simulated using RFPA2D-Basic V2.0 software. The SCB specimen had no prefabricated crack, and its radius was 25 mm. The loading rate was 0.000001 m/step. The results show that the fracture characteristics of the sandstone are affected by both the strength of the laminae and the angle; the fracture toughness and peak strength of the ultra-weak sandstone, as well as the weak sandstone, are reduced and more easily affected by the bedding angle; the strength rate of the strong sandstone is higher than that of the homogeneous sandstone, and the difference between the fracture characteristics of the two is not significant. This paper suggests that the key mechanism of this phenomenon is the anisotropy between the bedding and the sandstone, along with the competition/synergy between the main crack and the bedding plane bias crack during fracture propagation. These research results can provide a theoretical reference for the safety and stability of underground engineering in China.

STUDY OF SAND PRODUCTION STAGES IN SHALLOW OIL FIELDS

This article presents the High Pressure Consolidation System (HPСS) and a new experimental method for studying sand behavior in weak sandstone reservoirs. Under laboratory conditions, geostress conditions during the process of rock sedimentation, perforation, and sand production were replicated using a large artificial sample. A comprehensive investigation of sand production phenomena was conducted, where rock and pore pressures were fully controlled throughout the experiment using a computer. The HPCS also allows the study of single-phase and multiphase flow behavior in porous media at realistic reservoir pressures to model various well exploitation scenarios, such as implementing different depletion factors and water-oil ratios.

Determining static elastic modulus of weak sandstone in Andalusian historical constructions from non-destructive tests: San Cristóbal's stone

This work presents a relationship between static and dynamic elastic moduli for San Cristóbal's stone, which was used to build some of the most representative historical constructions in Andalusia (Spain) during 15th-18th centuries, including religious, military and civil buildings. Numerical models are able to provide useful information in structural health assessment of historical constructions, but static elastic modulus is necessary to perform them. This is why it is particularly interesting to count on an equation to predict this parameter from others, such as dynamic elastic modulus, which can be obtained in situ and through tests based on wave propagation.A new relationship is proposed after having shown that equations previously defined by other authors are not valid for San Cristóbal's stone. The proposed relationship in this work is based on a set of physical and mechanical experimental tests carried out in lab on 17 specimens directly extracted from support elements of Santiago's (Jerez de la Frontera, Cádiz-Spain). Linear, polynomial and nonlinear multiple regressions were considered, as well as the inclusion of other parameters, such as bulk density and porosity. However, an equation with a coefficient of determination of 0.95 was achieved with a simple regression where only dynamic elastic modulus was involved. This simple equation allows to predict static modulus of San Cristóbal's Stone with a high level of confidence and only from one parameter, that can be obtained in situ through non-destructive techniques and respectfully to built heritage.Finally, a first approximation to the application on an ancient construction is provided. Six columns of the Monastery of San Jerónimo de Buenavista, in Seville (Spain) underwent tests based on the propagation of wave to determine in situ their dynamic elastic modulus. The In situ results for the dynamic elastic modulus are consistent with those obtained in lab.

Numerical investigation of sand production mechanisms in weak sandstone formations with various reservoir fluids

A three-dimensional CFD-DEM simulation method was developed to investigate the influence of different reservoir fluids on sand production. The fluid properties and reservoirs were prescribed and modelled after the conditions in the Kenkiyak and Uzen oil fields in Kazakhstan. A cylindrical sandstone sample was modelled using the Discrete Element Method in three stages of the pluviation, diagenesis, and perforation processes as in the oil field. A coupled simulation with the Computational Fluid Dynamics was used to simulate the sand production from the perforated sandstone under different fluid flow conditions with light and heavy oils. A continuous sand production was found to be more severe with the heavy oil flow due to its higher transport capability and a microstructural sanding mechanism that mobilized a higher proportion of the sample's particles with the fluid outflow. Sand production for light oil occurred in two stages of a first transient sand production that was followed by a second continuous sand production. The transient sand production was associated with more significant bond breakage, particle movement and porosity change within the damage zone that was created by the central perforation inside the sandstone sample.

Role of plastic zone porosity and permeability in sand production in weak sandstone reservoirs

Sand production is often characterized as a two-stage process, in which material failure occurs near the cavity, leading to the formation of a plastic zone, from which particles are detached and transported out because of continuous hydrodynamic erosion under the effects of the produced fluid flow. The plastic zone porosity is affected by coupled processes, while the plastic zone permeability has a significant impact on the performance of sand production prediction, especially in weak sandstone reservoirs. Large-scale sand production experiments were conducted using a customized high-pressure consolidation apparatus. The results show that specific stress-fluid pressure conditions may create a plastic zone around a hole, which has lower permeability than the intact zone. The plastic zone comprises two subzones: a high-permeability shear band zone and a low-permeability compaction zone. During sand production, sand migrates from the compaction zone through the shear band zone to the perforation hole. Thus, sand production is associated with the increased permeability and porosity in the compaction zone. Existing sand prediction models were modified according to the new findings, resulting in a modified model with improved performance. The modified model was validated using the sanding data from a weak sandstone reservoir in Kazakhstan.

Determination of Seismic Site Class and Potential Geologic Hazards using Multi-Channel Analysis of Surface Waves(MASW) at the Industrial City of Abu Dhabi, UAE

ABSTRACT Geophysical investigation activities were conducted at ICAD-II, Abu Dhabi, UAE using Multi-Channel Analysis of Surface Waves (MASW) to determine subsurface geology , material stiffness, potential weak zones down to ~35 m depth, and to propose the appropriate seismic site classification for a proper foundation design. A total of 20 MASW lines were carried out over a grid layout of 5 m spacing. Data acquisition, processing and inversion have been parameterised and selected to produce shear velocities that represent subsurface conditions. The estimated average shear-wavevelocity (Vs30 = 577.97 m/s) suggests that the investigated site can be classified as Class C (V.D. Soil & Soft Rock). The constructed geological model comprises sand, weak sandstone, weak mudstone, and hard mudstone. Analysing the shear wave velocities indicates the absence of apparent cavities/ hazardous zone . However, a relatively weak layer of sandstone/mudstone rocks intercalations was detected from ~7m to ~25m. Meanwhile, the uncorrelated part of 2D MASW data indicates potentially harder mudstone encountered at a depth starting from ~25 m to >35 m. Hence, the foundation layer may be placed on the upper surface of the sandstone bed (~7 m) depending on the height and load of the proposed building and following the construction standards and requirements of the structural engineer.

Carbonate Rock Chemical Consolidation Methods: Advancement and Applications

Chemical consolidation of rocks is a common practice in the petroleum industry. Usually, this technique is applied to sustain production from unconsolidated or weak sandstone formations and to avoid collapse from aging wells. Recent research has shown the significance of rock hardness in sustaining long-term conductivity of hydraulic fractures in carbonate formations. There are some notable attempts to adapt the techniques used in the cultural heritage industry for strengthening carbonate formations. Moreover, a novel carbonate rock strengthening methodology was developed that involves the mineral transformation of calcite to harder minerals. This work describes the existing techniques for carbonate rock consolidation and provides extensive research on chemicals that are (or can be) used as carbonate rock strengthening agents. Furthermore, this review provides the reaction mechanisms associated with a range of proposed chemicals. Also, laboratory methods that can be used to assess post-treatment hardness and investigate the changes in rock mineralogy are summarized. Finally, the review discusses chemicals with the prospect of their application in petroleum field operations.

Numerical evaluation of Failure Assessment Diagram (FAD) for hydraulic fracture propagation in sandstone

This paper evaluates the degree of inelasticity in hydraulic fracture propagation in weak sandstone by using models obtained by the Discrete Element Method as a basis for quantifying Failure Assessment Diagram (FAD) and J-integral. Although current engineering models use Linear Elastic Fracture Mechanics (LEFM), including improvements like fracture tip damage zone where fracture tip plasticity is small enough to fit into the near-field stress zone, this paper hypothesizes that hydraulic fracture propagates in an inelastic regime in certain conditions typically for underground rock reservoirs. Discrepancies within field or laboratory data, including microcrack clouds identified as acoustic emissions that induce plastic deformations, motivate the investigation of inelasticity and implementation of elastoplastic fracture propagation theories. Discrete Element Method (DEM) has been widely used to approach rock failure problems from the micromechanical level, where explicit modeling of local particle-bond breakage enables insights into propagating fracture branching, damage, microcrack coalescence, stress–strain re-distribution, irreversible deformation, and fracture arrest. Results show that inelastic J-integral increases dramatically with rock confinement, especially its non-elastic portion, and the elastic portion of the total J-integral remains a relatively small part. Higher confinement stresses and higher confinement stress contrast enhance the inelastic fracture propagation. Additionally, shorter fracture lengths and smaller dry tip zones characterize inelasticity. Rock stiffness increase leads to total J-integral increase and decrease of J-elastic, leading to pronounced inelasticity. Therefore, results indicate that LEFM is rarely applicable for describing fracture propagation through rock at higher confinement stresses and stiffness.

Coupled CFD–DEM numerical modelling of perforation damage and sand production in weak sandstone formation

A coupled microscopic simulation method was implemented to investigate the bond breakage behaviour during perforation and sand production in a weak cemented sandstone. A new contact bond model was implemented in the open-source program CFDEM to simulate a large cylindrical sample of one hundred thousand numerical particles using a high-performance computer (HPC) system. The coupled simulation of the particle–fluid phases and the information exchange between them required intensive computational power and highly efficient parallelization algorithms, yet it provided the detailed information on the distribution of bond and bond breakage, new contact formation, particle migration and the associated change in the internal porosity across the sample. The perforation process was found to create a damage zone that contained a compacted core of unbonded particles. The compacted core expanded quickly under fluid flow condition and released its particles to migrate towards the perforation tunnel. Small particles were produced first from the perforation damage zone, while fewer particles of larger sizes were produced later in an overall transient sand production phenomenon.

Rock analysis to characterize Saudi soft sandstone rock

As more engineering projects and activities are taking place on and around weak rocks, it is becoming more important to study and characterize them. Since regular practices of rock mechanical testing are not effective for weak rocks, special laboratory tests and measurements were performed to characterize the Alkharj Saudi weak sandstone rock which is a clastic rock dominantly sandy limestone and sandstone. Test results are presented in this paper. Porosity, permeability, and mechanical properties (stress, strain, Poisson's ratio, confined compressive strength and unconfined compressive strength) were obtained and then used to characterize the proposed weak rock. This paper provides a mean of classifying weak soft rocks despite encountered problems in handling and testing such materials.

Modified Terzhagi’s Equation for Modulus of Subgrade Reaction

The structural design of substructure requires the modulus of a subgrade reaction of the underlying stratum at the soil-structure interface. Its magnitude is estimated by plate load tests at the site. The present study reviewed the magnitudes of the modulus of subgrade reactions obtained from plate load tests, elastic continuum solutions, and a soil-structure interaction analysis of a 2-km tunnel resting on weak sandstones and siltstones. The review showed that the structural adequacy of the substructure using the modulus of subgrade reaction obtained from the Terzaghi equation for plate load tests would not be satisfactory. A modified Terzaghi’s equation is presented so that the magnitude of the modulus of subgrade reaction obtained is of the same order of the magnitude as that obtained from the soil-structure interaction analysis and elastic continuum solutions. The proposed modified Terzaghi’s equation is applied to the plate load test results of two separate projects elsewhere to confirm its validity.

Coupled hydro-mechanical evolution of fracture permeability in sand injectite intrusions

Sandstone “injectite” intrusions are generally developed by the fluidization of weakly cemented sandstones and their subsequent injection into fractured reservoirs. In this work, a continuum coupled hydro-mechanical model TOUGH-FLAC3D is applied to simulate the discrete fracture network in large-scale sand injectite complexes. A sand production constitutive model is incorporated to consider the formation of sand through plastic deformation and its influence on evolution of fracture permeability. Overpressures in the fluidized sand slurry drives the injection with sand dikes intruded upwards, typically into previously low permeability “tight” mudstone formations. The contrast in poroelastic properties of the underlying weak sandstone and overlying injectite receptor directly affects the evolution of fracture aperture both during and after intrusion. Fluid drainage into the unconsolidated matrix may reduce the extent of fracture aperture growth, through the formation of shear bands. The results of this work have broad implications related to the emplacement of sandstone intrusions and subsequent hydrocarbon accumulation, maturation and then production.

Numerical investigation of the hydraulic fracturing mechanisms in oil sands

This paper presents a numerical investigation of hydraulic fracturing in oil sands during cold water injection by considering the aspects of both geomechanics and reservoir fluid flow. According to previous studies, the low shear strengths of unconsolidated or weakly consolidated sandstone reservoirs significantly influence the hydraulic fracturing process. Therefore, classical hydraulic fracture models cannot simulate the fracturing process in weak sandstone reservoirs. In the current numerical models, the direction of a tensile fracture is predetermined based on in situ stress conditions. Additionally, the potential transformation of a shear fracture into a tensile fracture and the potential reorientation of a tensile fracture owing to shear banding at the fracture tip have not yet been addressed in the literature. In this study, a smeared fracture technique is employed to simulate tensile and shear fractures in oil sands. The model used combines many important fracture features, which include the matrix flow, poroelasticity and plasticity modeling, saturation-dependent permeability, gradual degradation of the oil sands as a result of dilative shear deformation, and the tensile fracturing and shear failure that occur with the simultaneous enhancement of permeability. Furthermore, sensitivity analyses are also performed with respect to the reservoir and geomechanical parameters, including the apparent tensile strength and cohesion of the oil sands, magnitude of the minimum and maximum principal stress, absolute permeability and elastic modulus of the oil sands and ramp-up time. All these analyses are performed to clarify the influences of these parameters on the fracturing response of the oil sands.

A study on bond breakage behavior of weak Cretaceous Kazakhstani reservoir sandstone analogue

The study examines the effect of cement bond breakage in the mechanical behavior of porous weak sandstones in Kazakhstan’s oil fields using one-dimensional compression, triaxial shearing apparatuses, and Discrete Element Method (DEM) numerical simulation. Analogue sandstone materials were prepared by cementing a sand material with similar mineralogy to the real reservoir with the sodium silicate solution. The results of the cemented sandstones were compared to the results of the uncemented sand, disaggregated sandstone and silty sand of similar particle size distribution to understand the effect of the cemented structure. The failure behavior of the material was investigated in the experimental and numerical studies at different confining pressures and it was found that the behavior is mainly controlled by the cement bonds as there is no major change in the particles during the tests. It was found that the plastic yielding condition is mainly associated with the breakage of the cement bonds and this gives rise to volume compression and strain hardening of the material. The DEM results provide further confirmation that the degradation of the specimen stiffness is directly related to the bond breakage initiation and higher confining pressures affect the breakage rate and weaken the effect of the cemented structure, which is compensated by a stronger role for the frictional resistance between sand particles.

A sand production prediction model for weak sandstone reservoir in Kazakhstan

Weakly consolidated reservoirs are prone to sand production problem, which can lead to equipment damages and environmental issues. The conditions for sand production depend on stresses and properties of rock and fluid. Accurate sand volume estimation is, however, still a challenging issue, especially for reservoirs in weak formations. The weak reservoirs containing viscous or heavy oil are mainly discovered in shallow depths in Kazakhstan, with moderate temperature and pressure. Many prediction models developed for open-hole completions where the reservoir materials usually possess certain strength are not applicable for the local reservoirs where the materials are significantly weaker even if casing is used to support the wellbore with oil produced through the perforation tunnels. In this context, a prediction model was proposed where the volume of the produced sand was estimated as the volume of the plastic zone of the failed materials surrounding the perforation tunnels. The model assumes an evolving truncated conical shape for the damage zone and takes into account stress distributions and shear failure in this zone. Then, the proposed model was used to estimate sand volumes in 20 wells during oil production with sequential increase of flow rates. The predictions match well with the measured sand volumes in a local oil field. Finally, a sensitivity analysis was conducted on the model performance. It shows that the permeability of the plastic zone was the most significant controlling factor in the prediction results.

Compressive behaviour of very dense structured granular geo-materials

The isotropic compression behaviour of dense structured geo-materials and the associated degradation at failure is addressed in a non-qualitative manner. To this end, the general behaviour of fully de-structured geo-materials, e.g. sands, as an accepted reference is thoroughly investigated. The parameters affecting the behaviour of de-structured materials such as mineralogy, gradation and fines content, and relative density are discussed. The isotropic compression behaviour of a weathered weak sandstone, representative of a structured granular geo-material, is then investigated along the isotropic compression stress path under a range of pressures from nil to 100 MPa. Both structured and fully de-structured states of the material are tested implementing the proposed quantification method. The effect of structure on the compressibility of the material is found to be tangible. By plotting the specific volume versus natural log of the mean effective stress, the onset of structure collapse and the successive degradation of the structure are captured. By increasing the pressure, compaction bands throughout the sample increasingly develop and the compression curve asymptotically approaches to that of the fully degraded state of the material. At elevated pressures, the rate of compressibility will increase significantly due to particle crushing.

Fluid Properties Effects on Sand Production using Discrete Element Method

Oil production may be accompanied by Sand Production (SP) in the weak sandstone reservoirs. Fluid flow is an important factor in transporting the separated grains and completing the SP mechanism. In this paper, the effect of fluid parameters, fluid flow, and fluid pressure on the SP is investigated by applying the Discrete Element Method (DEM). Parametric studies show that fluid velocity is reduced by increasing the fluid viscosity, leading to a drop in the SP. In the present study, for an accurate investigation and discovering the effects of viscosity and drag force, the boundary conditions were applied to retain the fluid velocity as a constant amount. The results showed that viscosity is directly related to SP. Moreover, we found that when the fluid velocity is high, there would be the possibility of catastrophic production in the reservoirs with a heavy oil fluid. The rock reservoir around the well will be loosened as soon as SP is initiated. The results also indicate that SP has a direct relation with fluid pressure, fluid velocity, and confining pressure.