S.S. Rahman

An experimental investigation of hydraulic behaviour of fractures and joints in granitic rock

A method of measuring mean mechanical aperture of fractures based on gas volume balance is introduced. The effects of shear displacement and normal stress on mechanical and hydraulic behaviour of fractures are also investigated. The results obtained from tests conducted on granite samples from Olympic Dam (Central Australia) are compared with those calculated from existing shear dilation theories. It is found that hydraulic aperture is considerably narrower than the measured mean mechanical aperture. This highlights the need to consider tortuosity and surface roughness of fractures in the calculation of hydraulic aperture. It is also found that shear dilation angle decreases linearly with increases in confining pressure, as opposed to more rapid decreases predicted by existing empirical models. From the results of this study, a range of data describing the relationships between confining pressure, shear displacements, hydraulic aperture and permeability are presented, which could help to develop stimulation programs for geothermal reservoirs.

Hydraulic fracture initiation and propagation: roles of wellbore trajectory, perforation and stress regimes

This paper develops a generic model for predicting hydraulic fracture initiation from arbitrarily oriented wellbores. For a given in-situ stress condition and wellbore orientation parameters, the model predicts the fracture initiation pressure and the orientation and location of fractures on the wellbore wall. The model has been applied in a series of in-situ stress conditions to study the effect of wellbore orientation on fracture initiation using non-dimensional parameters, which have enhanced the applicability of presented results for any stress condition. Closed-form analytical solutions are also obtained for initiation of longitudinal, transverse and complex multiple fractures from vertical and horizontal wellbores with and without perforation. A numerical model is then incorporated in the study to analyze the propagation behavior of initiated fractures. Causes of fracture initiation at non-preferred locations and its effects on fracture propagation pressure and fracture volume due to twist of these initiated fractures during propagation are studied and discussed. Results from the analytical and numerical models used in this study are interpreted with a particular effort to enlighten the causes of abnormally high treating pressures during hydraulic fracture treatments.

Single and multiple transverse fracture initiation from horizontal wells

The results of an analytical and experimental study of the initiation of transverse fractures from horizontal wells are presented. Analytical criteria for the initiation of single hydraulic fracture are reviewed, and criterion for initiation of multiple hydraulic fractures was developed by modification of the existing Drucker and Prager criterion for single hydraulic fracture initiation. The developed criterion for multiple fracture initiation was validated by comparisons with actual hydraulic fracture initiation pressures, which were obtained from scaled laboratory experiments and numerical results from boundary element analysis. Other criteria are assessed against the experimental results. Experimentally obtained transverse fracture initiation pressures were found close to longitudinal fracture initiation pressures estimated from maximum tensile stress criterion and Hoek and Brown criterion. One possible explanation of this finding is presented. Results from Drucker and Prager criteria for single and multiple fracture initiation were, however, found closer to experimental values. Therefore, these criteria could be useful to engineers involved with hydraulic fracturing for predicting transverse fracture initiation pressures from horizontal wells drilled parallel to the minimum horizontal in-situ stress.

Analytical, numerical and experimental investigations of transverse fracture propagation from horizontal wells

This paper presents results of a comprehensive study involving analytical, numerical and experimental investigations into transverse fracture propagation from horizontal wells. The propagation of transverse hydraulic fractures from horizontal wells is simulated and investigated in the laboratory using carefully designed experimental setups. Closed-form analytical theories for Mode I (opening) stress intensity factors for idealized fracture geometries are reviewed, and a boundary element-based model is used herein to investigate non-planar propagation of fractures. Using the mixed mode fracture propagation criterion of the model, a reasonable agreement is found with respect to fracture geometry, net fracture pressures and fracture propagation paths between the modeled fractures and the laboratory tested fractures. These results suggest that the propagation of multiple fractures requires higher net pressures than a single fracture, the underlying reason of which is theoretically justified on the basis of local stress distribution.

Experimental investigation of shale membrane behavior under tri-axial condition

Abstract Wellbore instability in shale formations is one of the primary problems in oil and gas well drilling. The problem has been traditionally tackled by using oil-based drilling mud. However, this technique is costly and restricted by the environmental regulatory bodies. Recent studies have shown that borehole instability in shales can be managed by controlling the chemical potential of drilling mud. One of the critical issues in this approach is that shales are not ideal membranes. It is essential to understand the nonideal behavior of shale before the wellbore instability problem can be managed by the chemical potential approach. The nonideality of a shale membrane is, in general, a function of the type of the shale being drilled, composition of the formation water in the shale, burial depth of the shale, and chemical composition of the drilling mud used. In this paper, the theory on nonideal membrane was reviewed and identified for the purpose. The mathematical model was validated by experimental results obtained using a real field shale specimen from the Northwest Shelf of Australia. An example is also given to show how the model can be used to manage wellbore instability in shale by controlling the chemical composition of mud. The results of this study can be used as a useful guideline for formulating proper mud to drill troublesome shaly formations.

An integrated model for multiobjective design optimization of hydraulic fracturing

An integrated novel model for hydraulic fracturing design optimization is presented, which recognizes complex interactions between a hydraulically coupled fracture geometry module, a hydrocarbon production module and an investment-return cash flow module. Free design variables are identified and various design constraints are formulated, which must be satisfied so that an optimum design obtained is executable in the field using the specified surface equipment (pump, tubing, etc.) and that the treatment does not cause any undesirable formation damage by uncontrolled fracture growth and/or multiple secondary fracture initiation. The model is formulated within the framework of a multivariate and multiobjective optimization method, which is based on the combined features of Genetic Algorithm and Evolutionary Operation. A 2D fracture model is used to establish relationships between treatment parameters and fracture growth. The potential for hydraulic fracturing design improvement is demonstrated by application to a tight gas reservoir. Results show that the proposed model is instrumental in improving hydraulic fracturing design and achieving a goal-oriented optimum design in a conflicting environment. About 12% compromise with maximum possible production, or net present value (NPV), over 10 years can save up to 44% of initial hydraulic fracturing treatment cost. Furthermore, the 88% production, or net present value, as a result of an optimum treatment program is significantly higher than any arbitrary design. The issues of real-time design modification, however, are not included in this study.

Borehole collapse analysis incorporating time-dependent pore pressure due to mud penetration in shales

This paper presents a model to predict time-dependent instability of arbitrarily oriented wellbores. The underlying factor to make the borehole collapse time-dependent is the increase of pore pressure around the wellbore due to mud penetration over time. The time-dependent pore pressure around the wellbore is estimated by analyzing two-phase fluid flow in porous media. The petrophysical parameters required for two-phase flow analysis, such as capillary pressure and absolute and relative permeabilities are characterized from experimental data. The model for time-dependent borehole collapse analysis is then developed by incorporating the transient pore pressure in the widely used wellbore failure analysis model. Potential wellbore failure modes are mathematically defined and guidance is incorporated to identify the upper and the lower bounds of the safe mud window at a particular time. The developed model is applied to wells in shale under different stress regimes. Results demonstrate that an initial safe mud window becomes narrower as the time progresses. Consequently, a mud weight, which was initially safe, may become unsafe and results in borehole collapse after a certain period of time. Such a failure is further influenced by the orientation of the wellbore and different stress regimes.

Practical Application of Hybrid Modelling to Naturally Fractured Reservoirs

Abstract This article presents application of a hybrid method for modelling discrete fracture network in an actual naturally fractured reservoir (NFRs) (Palm Valley, Australia). The hybrid method integrates features of geological, statistical, artificial intelligence, and conditional hierarchical stochastic simulation techniques. Both discrete and continuum fracture information could be utilized, such as statistical distributions of fracture orientations, spatial distributions of fracture density, and discrete multi-fractal dimensions. The final output is a 3D network model of discrete fractures, with their corresponding details of location, size, and orientation. The results show an improvement of the hybrid method over previous fracture models.

Characterizing and Modelling of Fractured Reservoirs With Object-Oriented Global Optimization

Modelling of naturally fractured reservoirs is the first step to develop best scenarios for hydraulic fracture treatment, the design of an optimum production method and to evaluate reservoir potential. This paper reviews the state-of-the-art in current methods; hence, presents an integrated modelling methodology, utilizing object-based modelling, stochastic simulation and global optimization. Firstly, as an object-based model, each fracture is presented and treated as a discrete object. A stochastic simulation is carried out to generate an initial fracture network. An objective function is then formulated as the difference in statistics between the initial network and the target. Semi-variogram and other spatial statistical properties (cross variogram, multi-histogram mean and variogram distance) of fracture parameters are included so that the objective function is able to statistically describe representative field data. Subsequently, we use a global optimization algorithm to optimize the objective function. A case study is performed on an actual outcrop fault map to illustrate the proposed methodologys capacity. The results map the outcrop faults very closely.

Optimizing Treatment Parameters for Enhanced Hydrocarbon Production by Hydraulic Fracturing

A novel scheme is presented in this paper for hydraulic fracturing design, which integrates reservoir properties, operational limitations, fracture growth control requirements, reservoir production behaviour, and investment-return cash flow behaviour in deciding on the optimum values of various treatment parameters. The capability and robustness of the optimization scheme is demonstrated by applications to a tight gas reservoir for which various designs are obtained: maximum NPV design, maximum production design, a target production design, and a compromised design. Optimum designs are found to be different for different objective functions. It is demonstrated that maximization of NPV, or production, involves a high treatment cost, which can be minimized further by solving a combined objective function, but at the expense of some NPV or production. By trade-off analysis between production/NPV and treatment cost, 44% of treatment cost saving is indicated at the expense of only a 12% sacrifice in ptoduction/NPV. Various other design issues are investigated by sensitivity analyses.