Conductivity Vertical Fracture Research Articles

Practical pressure-transient analysis solutions for a well intercepted by finite conductivity vertical fracture in naturally fractured reservoirs

The semi-analytical or numerical solutions are the most used for pressure-transient analysis (PTA) for finite-conductivity fractured wells in naturally fractured reservoir now. However, the semi-analytical solutions are time-consuming and easily lead to inaccuracy at low fracture conductivity with less discretized segments. The numerical solutions are more time-consuming and usually face convergence and stability problems. These solutions are not convenient for practical well test analysis in Oilfield. Our motivation for this paper is to develop simply, quickly and stably analytical PTA solutions for finite-conductivity fractured wells in naturally fractured reservoir that can serve the well test analysis in Oilfield. We first derive a transformation method which can extend finite-conductivity fracture solutions in homogeneous reservoir to dual-porosity reservoir. Thereafter, analytical solutions for finite-conductivity fractured wells in homogeneous reservoir are presented. Then we provide a series of analytical finite-conductivity fracture solutions in dual-porosity reservoir under different boundary conditions based on the transformation method and homogeneous reservoir solutions. Six solutions given in the published literatures are used to verify the proposed solutions under different dimensionless fracture conductivity (FcD). Results indicate that (a) the proposed solutions are valid for FcD>1 with maximum error (5%) and this error will quickly decreases to around 1%; (b) the proposed solutions are applicable for infinite, closed and constant-pressure boundary conditions; (c) the proposed solutions can be extended to triple-porosity naturally fractured reservoirs after changing the f(s) function; (d) the proposed solutions are accurate and can greatly shorten the time of well test interpretation comparing to conventional solutions. Finally, the pressure-buildup data from Shunbei Oilfield is interpreted using the proposed solutions to verify its feasibility and practicability. The findings of this work can help for better understanding of its potential in quick well test analysis for field cases.

Transient responses of multifractured systems with discrete secondary fractures in unconventional reservoirs

This paper provides a semi-analytical model to analyze the transient responses of multifractured systems with discrete secondary fractures in unconventional reservoirs. The model idealizes the stimulated reservoir volume as two regions: a primary hydraulic-fracture and discrete secondary fractures. Transient pressure and derivative responses are obtained by coupling an analytical model for matrix flow and a numerical model for fracture flow. This approach can consider the fracture characteristics in detail. For example, the discrete secondary fractures may interconnect the primary hydraulic-fracture with different inclination, nonuniform fracture spacing and varying conductivity. The semi-analytical model was verified in comparison with the results of a fully numerical simulation model.Transient flow behavior of the system was analyzed in detail. The study has shown that a multifractured system with discrete secondary fractures may exhibit four flow regimes: bilinear flow, fluid feed flow, matrix linear flow and pseudosteady-state flow. In the fluid feed flow period, the secondary fractures act as supply sources to feed the primary hydraulic-fracture to generate a dip on the type curves. Local solutions for the matrix linear flow are similar to that for a finite conductivity vertical fracture and can be correlated with the ratio of the total length of the secondary fractures to that of the primary hydraulic-fracture and an intercept term. With the increasing of number of secondary fracture, the ending time of the bilinear flow will be advanced and the fluid feed flow period will come earlier. The dimensionless secondary fracture length determines the slope of the straight line of the matrix linear flow on square-root-dimensionless-time plot. As the secondary fracture inclination decreases, the duration of the matrix linear flow period is shortened and the beginning time of the pseudosteady-state flow period is delayed. In addition, the secondary fracture conductivity affects the depth of the dip and the beginning time of the matrix linear flow period. However, the influence is not appreciable on the log/log graph.

A new method in predicting productivity of multi-stage fractured horizontal well in tight gas reservoirs

The generally accomplished technique for horizontal wells in tight gas reservoirs is by multi-stage hydraulic fracturing, not to mention, the flow characteristics of a horizontal well with multiple transverse fractures are very intricate. Conventional methods, well as an evaluation unit, are difficult to accurately predict production capacity of each fracture and productivity differences between wells with a different number of fractures. Thus, a single fracture sets the minimum evaluation unit, matrix, fractures, and lateral wellbore model that are then combined integrally to approximate horizontal well with multiple transverse hydraulic fractures in tight gas reservoirs. This paper presents a new semi-analytical methodology for predicting the production capacity of a horizontal well with multiple transverse hydraulic fractures in tight gas reservoirs. Firstly, a mathematical flow model used as a medium, which is disturbed by finite conductivity vertical fractures and rectangular shaped boundaries, is established and explained by the Fourier integral transform. Then the idea of a single stage fracture analysis is incorporated to establish linear flow model within a single fracture with a variable rate. The Fredholm integral numerical solution is applicable for the fracture conductivity function. Finally, the pipe flow model along the lateral wellbore is adapted to couple multi-stages fracture mathematical models, and the equation group of predicting productivity of a multi-stage fractured horizontal well. The whole flow process from the matrix to bottom-hole and production interference between adjacent fractures is also established. Meanwhile, the corresponding iterative algorithm of the equations is given. In this case analysis, the productions of each well and fracture are calculated under the different bottom-hole flowing pressure, and this method also contributes to obtaining the distribution of pressure drop and production for every horizontal segment and its changes with effective fracture half-length and conductivity. Application of this technology will provide gas reservoir engineers a better tool to predict well and fracture productivity, besides optimizing transverse hydraulic fractures configuration and conductivity along the lateral wellbore.

The Water Absorption Rule of Vertical Fractured Well

Oil is an important energy and chemical raw materials and strategic materials. Nowadays, the layered water injection test technology become the key factor of oilfield production. According to different types of formation and for the artificial fracturing injection wells, this paper studied the infinite boundary, uniform flow and vertical cracks well, infinite diversion vertical fractures and conductivity vertical fracture wells’ absorbent law. The method to do all of the above work is to solve the equation of dimensionless bottomhole pressure in different formation and boundary conditions. The indicating curve of infinite uniform flow formation, unlimited conduction and limited conduction vertical fractures wells are almost identical, which means that the type of vertical fracture has little effect on the indicate curves of injection wells.

Numerical well test for well with finite conductivity vertical fracture in coalbed

A new well test model is developed for the hydraulic fractured well in coalbed by considering the following aspects: methane desorption phenomena, finite conductivity vertical fractures, and asymmetry of the fracture about the well. A new parameter is introduced to describe the storage of the fracture, which is named as a combined fracture storage. Another new concept called the fracture asymmetry coefficient is used to define the asymmetry of the fracture about the well. Finite element method (FEM) is used to solve the new mathematical model. The well test type curves and pressure fields are obtained and analyzed. The effects of the combined fracture storage, desorption factor, fracture conductivity, and fracture asymmetry coefficient on the well test type curves are discussed in detail. In order to verify the new model, a set of field well test data is analyzed.

Rate Decline Curves Analysis of a Vertical Fractured Well With Fracture Face Damage

Rate decline analysis is a significant method for predicting well performance. Previous studies on rate decline analysis of fractured wells are all based on homogeneous reservoirs rather than homogeneous ones considering fracture face damage. In this article, a well model intercepted by a finite conductivity vertical fracture with fracture face damage is established to investigate how face damage factor affects the productivity of fractured well. Calculative results show that in transient flow, dimensionless rate decreases with the increase of fracture face damage and in pseudo steady-state flow, all curves under different face damage factors coincide with each other. Then, a new pseudo steady-state analytic formula and its validation are presented. Finally, new Blasingame type curves are established. It is shown that the existence of fracture damage would decrease the rate when time is relatively small, so fracture damage is an essential factor that we should consider for type curves analysis. Compared with traditional type curves, new type curves could solve the problem of both variable rate and variable pressure drop for fractured wells with fracture face damage factor. A gas reservoir example is performed to demonstrate the methodology of new type curves analysis and its validation for calculating important formation parameters.

Practical Well Test Analysis of a Hydraulically Fractured Low Permeability Gas Reservoir: A Case History

The primary objective of hydraulic fracturing is to create a propped fracture with sufficient conductivity and length to amplify or at least optimize well performance of low permeability tight gas reservoir. The oil industry has suggested that hydraulically fractured tight gas wells performance is hindered significantly by non-Darcy flow effect.This work will present an investigation of non-Darcy flow effect to hydraulically fractured gas wells performance and provide the development, validation, and application of actual well test analysis for wells with a finite conductivity vertical fracture.Also, this work presents the results obtained in the study of actual post frac modified isochronal test data of gas wells intersected by a finite conductivity vertical fracture in a tight low permeability gas reservoir. In addition, the estimation of reservoir properties and fracture properties were carried out to construct a simple analytical model, which used for rate prediction.The effect of non-Darcy flow in fractures is clearly seen in the tests data and will lead to limiting production especially on higher chokes (after one inch). Therefore, non-Darcy effects should be considered in design of hydraulic fracture treatments, otherwise the design might be far from optimal

Analysis and evaluation of gas well tests for wells intersected by finite conductivity hydraulic fractures

A single-phase two-dimensional mathematical model was used for analyzing the behavior of a gas well intersecting a finite conductivity vertical fracture at the center of a closed square gas reservoir. Turbulent flow in both the fracture and the formation was included in the model. The analysis of the simulated well tests at constant mass rate at the sand face showed that the early portions of log m( p) vs. log t f plots do not display the characteristic one-half slope line when the dimensionless fracture conductivity ( F CD ) is less than 416. Instead, the plots display much smaller slopes, regardless of fracture length, flow rate, formation permeability, and initial reservoir pressure. The time required to reach the radial behavior constant slope line on m( p wf) vs. log t f plot was found to increase with: (1) increasing fracture conductivity ( F CD); (2) increasing fracture length; (3) decreasing formation permeability; and (4) increasing flow rate. Turbulent flow is found to be most significant inside the fracture. The square-root time method [ m(p) vs. t μc g ] during the linear flow period yields a good estimate of the quantity kx 2 when fracture conductivity ( F CD)⩾416. In addition, this method could be used to estimate kx 2 when F CD>5 if the linear flow period inside the fracture can be defined. Finally, this paper shows that high-conductivity fractures are more important as a design criterion than the length of the fracture.

Evaluation of Production Tests in Oil Wells Stimulated by Massive Acid Fracturing Offshore Qatar

McDonald, Simon W.; SPE, Qatar General Petroleum Corp. (Offshore Operations) Summary This paper presents the evaluation of pressure-buildup data from production tests in wells that have been stimulated by massive acid fracturing. Fracture type curves are used in combination with conventional semilog analysis techniques. Fracture characteristics are calculated from a match of the early-time pressure data with the type curves, and reservoir characteristics are calculated from a conventional semilog plot of late-time data. Unexpectedly high formation permeabilities are evaluated, and fracture half-lengths are much shorter than design values. Introduction The Idd el Shargi field lies about 80 km offshore Qatar and consists of two culminations, called the North and South Domes, separated by a saddle. The Shuaiba reservoir is the shallowest oil-bearing reservoir in the field. It lies 4,500 to 5, 100 ft subsea and contains large volumes of oil trapped in predominantly low-permeability, chalky reservoir rock. Most of this oil is trapped in the North Dome. Offtake from the North Dome Shuaiba over the past 17 years has been primarily from three wells located near the crest of the structure where leaching has enhanced reservoir permeabilities and where well PI's are high. ranging from 10 to 60 B/D/psi. The depletion rate of the reservoir through these three wells historically has been very low. To date there has been no sustained off-take from the South Dome Shuaiba, and therefore both these reservoirs offer considerable development potential. Most of the Shuaiba chalk is low-permeability reservoir rock, with nonimpaired (i.e., zero skin factor) well PI's calculated to be about 0.1 B/D/psi or less. One of the most significant factors about a potential secondary-recovery development scheme for this reservoir is the stimulation necessary to promote flow from or injection into the wells at acceptable rates. In recent years, massive acid fracture (WISPER-type-widely spaced etched ridges) stimulations have been carried out in selected pilot wells. Subsequent postfracturing production performances of these wells have proved the suitability of this process for stimulating the soft, low-permeability Shuaiba chalk. This paper describes the analysis method employed by Qatar General Petroleum Corp. (Offshore Operations) to interpret the bottomhole pressure (BHP) buildup data from production tests carried out following acid fracturing the Shuaiba reservoir in the pilot wells. Of particular interest has been the calculation of the formation permeability thickness, kh, and fracture half-length, Xf, from these tests, as these will have a major impact on the design of a possible future secondary-recovery scheme. Theory Recently several papers have presented pressure drawdown data for various types of uniform influx, infinite conductivity, and finite-conductivity vertical fractures within a homogeneous isotropic formation. The data generally are presented in the form of log-log graphs (type curves) of the dimensionless pressure drop, PwD, vs. dimensionless time, tDxf, for the different fracture types. JPT P. 496^