Optimist, Pessimist or Engineer - Conductivity Based on Need Not Fashion

An optimist says the glass is half-full, a pessimist half-empty, whereas a good engineer says that the glass is twice as big as it needs to be. There has been much debate over the years about the relative functionality, application and even necessity of proppant in delivering effective hydraulic fractures. Often these debates have been directly linked to major changes in core frac applications, more recently in the dominant North American onshore unconventional market. However, the debates have all too often used broad or unclear brush strokes to describe shifting fracture requirements. Meanwhile, the developing oilfield in the rest of the world resides in more permeable areas of the resource triangle, great care must be taken to ensure that conventional lessons hard learned are not lost, but also that unconventional understanding develops. Over recent years there have been many debates and publications on the relative value of the use of proppant (and associated conductivity), although the true question was about appropriate fracture design in different rock/matrix qualities and environments. Certainly, the vast majority of fracturing engineers appreciate the difference between continuous proppant-pack conductivity and other techniques, such as infinite conductivity, pillar fracturing or duning designs. However, there is increasing evidence that conventional fracturing is suffering from populist attitudes, leading to ineffective fracturing. Additionally, and just as impactful, that unconventional fracturing continues to rely on the lessons learned and physics derived directly from our conventional experience but applying this in an entirely different environment. Primarily, the main concern is with the transfer of recent lessons learned and techniques utilised in one rock quality and environment, to an entirely different scenario, resulting in the misapplication, reduced IP30, poorer NPV or reduced long term EUR and IRR. Examples will be referenced where appropriate proppant selection and frac design can be the difference between success and failure. Fundamentally, we have not sufficiently developed our understanding of the role of proppant and conductivity, for application in unconventionals and thereby rely far too much on our previous conventional thinking. While at the same time we are exporting often inappropriate unconventional populist practice into very conventional environments, thereby potentially achieving the abhorrence of the worst of both worlds. This paper will describe and address scenarios where appropriate engineering selection, rather than popularity-based decision making, has resulted in a successful outcome. It will also attempt to ensure that we show the importance of studying your rock, in anticipation of engineering design, and that this should be a key consideration. The paper will also suggest that as an industry we urgently need to address our approach to consideration of conductivity, placement and importance and ensure that unconventional knowledge and learning progresses with a beneficial outcome for all.

Pillar Fracturing Production Enhancement Results for an Unconventional Calcareous Shale in Ecuador

Economical production from low-permeability oil-saturated reservoirs has always been a challenge in a basin known for its mature assets. M2 limestone is a new challenge. To characterize, it was necessary to use the methodology based on shale plays, integrating information from different logs using a proprietary evaluation method. Applying pillar fracturing, creating stable voids between pillars, and hence, infinite-conductivity channels in geomechanically competent candidates resulted in economical production and proved reserves from a low-permeability calcareous shale. Geomechanics, mineralogy, and saturated intervals were addressed by using a combination of rock mechanical properties and mineralogy, carbon/oxygen logs, and X-ray diffraction (XRD) on drilling cuttings. Once the prospective zones in the M2 limestone intervals were selected, a conventional fracturing treatment was designed using a 3-D gridded simulator. The candidate well was evaluated for pillar fracturing by using results from geomechanics and the conventional fracture application. A pumping schedule that included pillar volume, spacer, and tail in stages was then designed. Results from the fracture simulator were loaded in a numerical reservoir simulator, and different development scenarios were evaluated. M2 limestone has shown production potential near areas where volcanic intrusion is present, or indicated hydrocarbon potential by oil shows observed on cuttings and high-gas readings during drilling. The data used for this project was collected during conventional reservoir development but had never been evaluated using an unconventional reservoir approach. XRD analysis and acid solubility tests confirmed that the reservoir does not contain a high-carbonate content nor acid solubility. Diagnostic Fracture Injection Test (DFIT) and minifrac analysis helped to define the size and fracturing technique to be used. Results from this work provided a better understanding of the reservoir; a development plan is needed to improve the investment return for this type of project. Geomechanical evaluation is fundamental to the application and design of pillar fracturing. This fracturing technique was selected because it used 43% less proppant than a conventional job, reduced risk of screen out, and provided higher productivity over a conventional fracturing job. This is the first time that pillar fracturing has been applied in this Ecuadorian reservoir. The production outcome proved reserves of 32°API oil and resulted in the largest fracturing job in Ecuador. Different development scenarios are proposed based on the results from this well. A complete workflow to characterize, design a hydraulic fracture job using proprietary geomechanical candidate selection criteria, and develop an unconventional calcareous shale is presented. The available data are the same as in a conventional reservoir, whereas the evaluation technique, as well as fracture design, is customized to this type of reservoir to attain economical production.

Disorder-Induced Superconductor-Insulator Transition

In this paper, we report the results of our theoretical investigation on the interplay of superconductivity and disorder in two-dimensional (2D) systems. The effect of disorder on superconductivity of 2D systems was found analytically using Green’s function formalism. The results of our calculation revealed that disorder induced due to randomly distributed superconducting islands enhances decoherence of Cooper pairs and suppresses superconductivity. We have also determined the critical value of disorder at which the 2D system completely loses its superconducting properties. Below this critical value of disorder, the system acts as a superconductor, a system with zero electrical resistance. Above the critical value, it acts as an insulator, a system with infinite electric resistance. This is a fascinating result because a direct transition from the state of the infinite conductivity to the opposite extreme of infinite resistivity is unexpected in the theory of condensed matter physics.

The Dual-Reciprocity Boundary Element Method Solution for Gas Recovery from Unconventional Reservoirs with Discrete Fracture Networks

Summary In this paper, we present a novel application of the dual-reciprocity boundary-element formulation (DRBEM) to model compressible (gas) fluid flow in tight and shale-gas reservoirs containing arbitrary distributed finite- or infinite-conductivity discrete fractures. Compared with the standard boundary-element method (BEM), the DRBEM transforms the nonlinear domain integrals at the righthand side (RHS) of BEM formulations for nonlinear partial differential equations into equivalent boundary integrals. This transformation allows retention of the domain-integral-free, boundary-integral-only character of standard BEM approaches. The proposed approach is based on coupling DRBEM with the finite-volume method (FVM) in which a multidimensional system is solved by integrating over a line with random fractures. The resulting system of equations is solved simultaneously for fracture and matrix boundary conditions by combining DRBEM and FVM without invoking any approximation for pressure-dependent nonlinear terms such as gas viscosity and compressibility. Numerical examples and field cases are presented to test the validity and showcase the capabilities of the proposed approach. The proposed model provides a general framework that can be applied to a variety of well and fracture geometries and operating schedules, and it is used to analyze production behavior for these complex systems. To the best of the authors’ knowledge, this is the first successful application of the dual-reciprocity principle to the BEM analysis of massively fractured horizontal wells (MFHWs) performance in natural-gas formations in which nonlinear, pressure-dependent gas properties are captured without approximation.

Well-flow behavior of reorientation fracture considering permeability anisotropy

Purpose In petroleum industry, hydraulic fracturing is essential to enhance oil productivity. The hydraulic fractures are usually generated in the process of hydraulic fracturing. Although some mathematical models were proposed to analyze the well-flow behavior of conventional fracture, there are few models to depict unconventional fracture like reorientation fracture. To figure out the effect of reorientation fracture on production enhancement and guide the further on-site operating, this paper aims to investigate the well-flow behavior of vertical reorientation fracture in horizontal permeability anisotropic reservoir. Design/methodology/approach Based on the governing equation considering horizontal permeability anisotropy, the mathematical models for reorientation fractures in infinite reservoir are developed by using the principle of superposition. Furthermore, a rectangular closed drainage area is also considered to investigate the well-flow behavior of reorientation fracture, and the mathematical models are developed by using Green’s and source functions. Findings Computational results indicate that the flux distribution of infinite conductivity fracture is uniform at very early times. After a period, it will stabilize eventually. High permeability anisotropy and small inclination angle of reorientation will cause significant end point effect in the infinite conductivity fracture. The reorientation fractures with small inclination angle in high anisotropic reservoir are capable of improving 1-1.5 times more oil productivity in total. Originality/value This paper develops the mathematical methods to study the well-flow behavior for unconventional fracture, especially for reorientation fracture. The results validate the production enhancement effect of reorientation fracture and identify the sensitive parameters of productivity.

A Novel Model Incorporating Geomechanics for a Horizontal Well in a Naturally Fractured Reservoir

Fracture aperture of a fractured reservoir can be affected by both matrix elasticity and fracture compressibility when the reservoir pressure decreases, namely stress sensitivity. An elasticity parameter coupling Young’s modulus and Poisson’s ratio was introduced to reflect this geomechanical behavior, and a new model incorporating geomechanics was developed to analyze the flow behavior of a horizontal well in a naturally fractured reservoir. Pressure solutions for two cases—uniform-flux and infinite-conductivity—were derived, respectively. For the uniform-flux case, the effect of dimensionless elasticity parameter on the pressure-drop profile becomes stronger with continuing production, and the profile may be like a bow. Nine flow regimes can be observed on the transient response of the infinite-conductivity case. Stress sensitivity mainly affects the late-flow period and a larger dimensionless elasticity parameter causes a greater pressure drop. Due to stress sensitivity, the pressure derivative curve exhibits an upward tendency in the pseudo-radial flow regime, and the slope is greater than “1” in the pseudo-steady flow regime. For KT-I formation in the North Truva field, its elasticity parameter decreases with the increase of Young’s modulus or Poisson’s ratio and ranges from 8 × 10−8 Pa−1 to 1.1 × 10−7 Pa−1. Meanwhile, the transient response of H519 has a slight negative correlation with Young’s modulus and Poisson’s ratio in the pseudo-steady flow regime.

PRIDICTION OF THE PRODUCTIVITY OF HYDRAULIC FRACTURED VERTICAL WELL WITH INFINITE CONDUCTIVITY FRACTURE

Hydraulic fracturing is used to significantly improve well productivity, particularly in reservoirs with low permeability. In carrying out this operation in the reservoir creates a large branched system of cracks. Increase filtration connection of the wells to remote areas. This article will present the results of numerical solution of the problem related to the definition of increasing the productivity of vertical wells after hydraulic fracturing, with infinite conductivity fracture. As a result, productivity index increases with penetration ratio distance for all aspect ratios.

Enhanced-Oil-Recovery Potential for Lean-Gas Reinjection in Zipper Fractures in Liquid-Rich Basins

Summary Production from liquid-rich shale has become an important contributor to US production, but recovery factors are low. Enhanced-oil-recovery (EOR) methods require injectivity and interwell communication on reasonable time scales. Herein, we investigate the development of fracture interference for the application of recycled-lean-gas injection to displace reservoir fluids between zipper fractures in liquid-rich shales. In condensate systems, the liquids produced from miscible displacement could be extracted at the surface and the gas reinjected. In unconventional oil systems, immiscible displacement would occur with arrest in the oil-rate decline upon the onset of pressure support until immiscible front breakthrough, although this may never occur in a reasonable time. In either case, the time for interference is critical in assessment of process feasibility. Using superposition plus existing analytical solutions to the diffusivity equation for arbitrarily oriented line sources/sinks for pressure and new extensions for the pressure logarithmic temporal derivative, we analyze the time for interfracture-communication development (i.e., interference) and productivity index (PI) for both classical biwing fractures in a zipper configuration and complex-fracture networks. As a novel contribution, we demonstrate the ability to map both pressure and pressure temporal derivative as a function of time and space for production and/or injection from parallel motherbores under the infinite-conductivity wellbore and fracture assumption. The infinite-conductivity assumption could be relaxed later for more-general cases. We present the results in terms of geometrical-spacing requirement for both horizontal wells and stimulation treatments to achieve reasonable time frames for interfracture communication and sweep for parameters typical of various shale plays. Results can be used to determine whether spacing currently considered for primary production is sufficient for direct implementation of EOR or if current practice should be modified with EOR in the field-development plan.

Optimization of Spacing and Penetration Ratio for Infinite-Conductivity Fractures in Unconventional Reservoirs: A Section-Based Approach

Summary In this paper, we consider the development plan of shale gas or tight oil with multiple multistage fractured laterals in a large square drainage area that we call a “section” (usually 640 acres in the US). We propose a convenient section-based optimization of the fracture array with two integer variables, the number of columns (horizontal laterals) and rows (fractures created in a lateral), to provide some general statements regarding spacing of wells and fractures. The approach is dependent on a reliable and efficient productivity-index (PI) calculation for the boundary-dominated state (BDS). The dimensionless PI is obtained by solving a time-independent eigenvalue problem by use of the finite-element method (FEM) combined with the Richardson extrapolation. The results of the case study demonstrate two decisive factors: the dimensionless total fracture length, related to the total amount of proppant and fracturing fluid available for the section, and the feasible range of actual fracture half-lengths, related to current fracturing-technology limitations. Under the constraint of dimensionless total fracture length, increasing the number of columns (horizontal laterals) increases the total PI but with only diminishing returns, whereas the optimal fracture-penetration ratio decreases somewhat, but is still near unity. When adding the technological constraint of a limited range of fracture half-lengths that can be routinely and reliably created, only a few choices remain admissible, and the optimal decision can be easily made. These general statements for the ideal homogeneous and isotropic formation can serve as a reference in the more-detailed optimization works. In other words, we offer a first-pass method for decision making in early stages when detailed inputs are not yet available. The information derived from the section-based optimization method and the efficient and reliable algorithm for PI calculation should help the design of multistage fracturing in shale-gas or ultralow-permeability oil formations.