Well-Test Analysis for Naturally Fractured Reservoirs

Author:

Cinco-Ley Heber1

Affiliation:

1. CSIPSA and UNAM

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Summary Well testing provides a powerful tool to detect and to evaluate heterogeneities in naturally fractured reservoirs (NFR's). Experience has shown that this type of reservoir may display behavior that consists of a variety of flow models. This paper presents a discussion of the applications and limitations of pressure-transient tests in the evaluation of NFR's. Introduction Optimizing the exploitation of a reservoir requires a complete description of the formation. A combination of information from various sources allows a reliable characterization of the system so that data from seismology, geology, well logging, well tests, core and fluid analysis, and well flow rates can be used to estimate reservoir geometry, oil and gas in place, and flow characteristics of the porous medium among other factors. Well testing1–10 provides an ideal tool to find reservoir-flow parameters and to detect and evaluate heterogeneities that affect the flow process in the formation. NFR's contain a considerable amount of the world hydrocarbon reserves. AsFig. 1 shows, the rock in these types of systems may include several elements (i.e., vugs, fractures, and matrix). Hydrocarbons are contained in both fractures and rock matrix usually, fractures act as channels to yield highwell-flow rates. Reservoir studies must consider these heterogeneities because they can affect oil and gas recovery significantly. Because of the importance of NFR's, many publications haye appeared that provide an understanding of the behavior of these types of reservoirs.11,12 Currently, advances in well-test analysis allow a more reliable characterization of these systems based on new flow models that properly account for heterogeneities of fractured reservoirs. Owing to space limitations, discussing every publication that contributed to the technology used is impossible. Experience has shown that NFR's may behave according to a variety of reservoir-flow models:homogeneous reservoir,multiple region or composite reservoir,anisotropic medium,single fracture system, anddouble-porosity medium. Fig. 2 shows the main elements of these models, and Table 1 shows the different sets of parameters that have to be found to describe the flow behavior in the reservoir for each case. Next, we discuss the application and limitations of these models in well-test analysis. Homogeneous Reservoir Model. This model considers that reservoir properties are constant and do not vary throughout the reservoir. Fractures and rock matrix act as a single medium so that fluid production is caused by the simultaneous expansion of both elements, and fluid transfer between them, if any, occurs instantaneously without resistance. This behavior is exhibited by either a heavily fractured reservoir with small matrix blocks (Fig. 2a) or by an NFR where fluids are contained mainly in the fracture system (Fig. 2b). The presence of fractures can be detected by the analysis of well logs and cores. In general, well-test-analysis methods have been developed for homogeneous reservoirs. The pressure behavior in these systems is controlled by the formation flow capacity, kh; porosity f, fluid viscosity µ, and total compressibility, ct. An essential part of well-test-analysis methods is a flow-regime diagnosis achieved through the application of alog-log graph of both pressure and pressure derivative.13 This process allows the detection of flow geometries and the presence of heterogeneities in the reservoir. The parameters of the system are estimated by use of the specialized graphs of pressure, p, vs. time, t (e.g.,p vs. log t, p vs. t1/2, p vs.t1/4, p vs. t-1/2,p vs.1/t, and p vs. :t, correspending to radial,14linear,15 bilinear,16 spherical,17 constant pressure boundary,18 and pseudosteady-state2 flows, respectively. Fig. 3 illustrates the behavior of a single-well test (drawdown orbuildup) for radial flow in homogeneous systems. The first part of the pressure-derivative graph (unit-slope straight line) shows the presence of wellbore-storage effects followed (after a transition zone) by a horizontal portion representing a radial-flow behavior. The kh of the formation and the skin factor can be determined from the straight line on the semilog graph. The estimated kh for an NFR represents an equivalent value for the fracture/matrix system. As a complement to single-well tests, interference tests are used to estimate the storativity of the formation (fcth); this value is usually high for NFR's because it includes the storativity of both fractures and matrix. Sometimes a high-productivity well produces from a reservoir with small hydrocarbon reserves; here, the fluids are contained mainly in the fractures and the system behaves as a homogeneous medium. Extended drawdown tests allow the early detection of this case during the appraisal phase of the reservoir. Multiple Region or Composite Reservoir Model. Some NFR's are fractured regionally. (Fig. 2c) and can therefore be considered to be composed of two regions: a high- and a low-transmissibility region. In this case, the reservoir behaves as a composite radial system.19 Wells producing from the fractured region exhibit higher productivity than those in the unfractured region. The system is characterized by the flow capacity of both regions (kh)1 and(kh)2.

Publisher

Society of Petroleum Engineers (SPE)

Subject

Strategy and Management,Energy Engineering and Power Technology,Industrial relations,Fuel Technology

Cited by 22 articles. 订阅此论文施引文献 订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献

同舟云学术

1.学者识别学者识别

2.学术分析学术分析

3.人才评估人才评估

"同舟云学术"是以全球学者为主线,采集、加工和组织学术论文而形成的新型学术文献查询和分析系统,可以对全球学者进行文献检索和人才价值评估。用户可以通过关注某些学科领域的顶尖人物而持续追踪该领域的学科进展和研究前沿。经过近期的数据扩容,当前同舟云学术共收录了国内外主流学术期刊6万余种,收集的期刊论文及会议论文总量共计约1.5亿篇,并以每天添加12000余篇中外论文的速度递增。我们也可以为用户提供个性化、定制化的学者数据。欢迎来电咨询!咨询电话:010-8811{复制后删除}0370

www.globalauthorid.com

TOP

Copyright © 2019-2024 北京同舟云网络信息技术有限公司
京公网安备11010802033243号  京ICP备18003416号-3