Ultra-low Permeability Gas Reservoirs Research Articles

Pressure performance of multi-stage fractured horizontal well considering stress sensitivity and dual permeability in fractured gas reservoirs

Generally, the horizontal well multi-stage fracturing technology is widely used to enhance single well production of tight or ultra-low permeability gas reservoirs. The deformation of the rock pore structure makes the permeability of the reservoir impossible to be a constant. The pressure transient analysis mathematical model of the multi-stage fractured horizontal wells (MFHWs) with consideration of stress sensitivity, varying conductivity and dual-porosity dual-permeability (DPDP) is established. Laplace transformation, pressure drop superposition and Gaussian elimination are utilized to obtain the wellbore pressure solution of MFHWs. Comparison result reaches a good agreement by model validation. On the basis of pseudo-pressure derivative curve characteristics, eight flow regimes can be diagnosed, namely, well storage, skin affection, bi-linear flow, early linear flow, early radial flow, middle linear flow, inter-porosity flow and later radial flow regime. The influences of some vital parameters on pseudo-pressure and its derivative curve is discussed in detail. The proposed model can help engineers understand the pressure dynamic characteristic of the MFHWs with stress sensitivity, varying conductivity and DPDP performance.

Sequentially coupled flow and geomechanical simulation with a discrete fracture model for analyzing fracturing fluid recovery and distribution in fractured ultra-low permeability gas reservoirs

More accurate characterization and prediction of the in-situ distribution of fracturing fluid in fractured reservoirs are needed for enhancing well productivity. In this study, an implicit-sequentially coupled flow/geomechanics simulator incorporating an efficient discrete fracture model is developed to model fluid distribution and recovery performance of ultra-low permeability gas reservoirs. The finite-volume and finite-element methods are used for space discretization of the flow and geomechanics equations, respectively, while the backward Euler method is employed for time discretization. The flow and geomechanics equations are solved sequentially based on fixed-stress splitting. An efficient discrete-fracture model is used to explicitly model the fractured system. Flexible unstructured gridding is employed to model arbitrarily-oriented fractures. The interrelations among pore volume, permeability and geomechanical conditions are considered dynamically using two-way coupled flow and geomechanics computations.The geometry of fracture (networks) due to hydraulic fracturing has significant impacts on the fracturing fluid recovery efficiency and ensuing fluid distribution. Under the same injection volume, the fracturing fluid recovery is higher when the fracture geometry is planar. Fluid recovery is relatively lower whenever natural fractures are activated during fracturing treatments; flowback time is also shortened when complex fracture network with enlarged fracture interface is present. Fracturing fluid in hydraulic fractures may leak off into the natural fractures and subsequently imbibes into the surrounding matrix due to capillarity effects. The fracturing fluid recovery and in-situ fluid distribution are sensitive to the shut-in duration and fracture closure behavior.This study analyzes the coupled flow-geomechanical responses of fractured gas reservoirs during the post-fracturing periods. Understanding the fate of the fracturing fluid can provide insights on, to some extent, the stimulated fracture volume, size of the water invasion zone, and efficiency of the fracturing design. The simulation predictions can also provide more accurate initial reservoir conditions (e.g. distributions of different phases and pressure) for long-term well performance estimation.

Productivity model for gas reservoirs with open-hole multi-fracturing horizontal wells and optimization of hydraulic fracture parameters

Abstract Multi-fractured horizontal wells are commonly employed to improve the productivity of low and ultra-low permeability gas reservoirs. However, conventional productivity models for open-hole multi-fractured horizontal wells do not consider the interferences between hydraulic fractures and the open-hole segments, resulting in significant errors in calculation results. In this article, a novel productivity prediction model for gas reservoirs with open-hole multi-fractured horizontal wells was proposed based on complex potential theories, potential superimposition, and numerical analysis. Herein, an open-hole segment between two adjacent fractures was regarded as an equivalent fracture, which was discretized as in cases of artificial fractures. The proposed model was then applied to investigate the effects of various parameters, such as the angle between the fracture and horizontal shaft, fracture quantity, fracture length, diversion capacity of fractures, horizontal well length, and inter-fracture distance, on the productivity of low permeability gas reservoirs with multi-fractured horizontal wells. Simulation results revealed that the quantity, length, and distribution of fractures had significant effects on the productivity of low permeability gas reservoirs while the effects of the diversion capacity of fractures and the angle between the fracture and horizontal shaft were negligible. Additionally, a U-shaped distribution of fracture lengths was preferential as the quantity of fractures at shaft ends was twice that in the middle area.

Classification of Horizontal Wells Based on Dynamic Data and its Application in Ultra-Low Permeability Gas Reservoirs

A scientific and well-established approach to evaluation and classification of gas wells plays an important role in management of low-permeability gas reservoirs. In this paper, we present a new classification system for horizontal wells, based on dynamic data from 40 wells in the Su-14 block of the Sulige field. We also introduce such evaluation parameters as the drop in well production with a decrease in casing pressure, the casing pressure drop rate, and absolute open flow. We propose using cluster analysis for evaluating the performance of Su-14 wells and for classification of the wells. Analysis of three wells has confirmed that the proposed method is reliable and accurate. This classification method is of considerable theoretical and practical value, and can be used for other similar gas fields.

Practical insights into liquid loading within hydraulic fractures and potential unconventional gas reservoir optimization strategies

The U.S. has experienced a resurgence of the upstream hydrocarbon sector in recent years, owing to the economic extraction of oil and gas from ultra-tight reservoirs using multistage hydraulic fracturing in horizontal wells. This success is often attributed to slick-water stimulation treatments that help create extensive complexity and contact with the low permeability reservoir. In this process, hundreds of thousands of barrels of water are pumped downhole, along with friction reducers, low concentration linear gel, fracture propping sand and other additives, to create and sustain these fractures. However, only a small percentage of this stimulation water is recovered back once the well is put back on production. This not only leads to excessive water hauling costs for operators in each consecutive well but also liquid blockage for hydrocarbon flow. Such water blockage/loading may become a serious concern in dry gas reservoirs such as the Marcellus field in the northeastern U.S., due to the unfavorable hydrocarbon mobility ratios. In spite of its implications on early and late time well performance, the issue of hydraulic fracture cleanup and gas flowback through it when drained through a horizontal wellbore is still an insufficiently understood subject. In this study the authors investigate the potential of liquid loading (stimulation water or condensate) within the hydraulic fracture itself due to low matrix permeability and insufficient drawdown conditions. Similar conditions may also arise late in the life of well when the reservoir pressure has declined significantly or due to wellbore design issues. A 3D reservoir simulation model with a discrete, planar hydraulic fracture is set up to investigate the competition between capillary, viscous and gravity forces within the fracture. The results indicate a strong tendency for liquid loading in the ultra-low permeability gas reservoirs under common operational constraints and offer recommendations on best practices to minimize its impact.

Application of the unstructured grids in the numerical simulation of fractured horizontal wells in ultra-low permeability gas reservoirs

Given the impermeable lithological boundary problem of the Sulige Gas Field in China, this study validates the dynamic characteristics of this reservoir through a radial model of the vertical well and proposes an unstructured grid refinement technology using a perpendicular bisector, while considering the horizontal well fracturing model. The vertical fractures and impermeable lithological boundary around the horizontal well can be accurately simulated using the unstructured grid refinement technology. This study also investigates the influence of stress sensitivity on the development indices of an ultra-low permeability gas reservoir. Results show that an impermeable lithological boundary in a horizontal well can effectively decrease the production rate of the working system of intermittent open wells. For ultra-low permeability gas reservoir with different stress sensitivities, important aspects, such as stabilization period, economic benefits, and ultimate recovery, should be considered in the early production allocation of gas wells.