Abstract

A newly developed mathematical model has been used for formation damage analyses of two hydraulically fractured horizontal wells in the Daqing field, China, and twelve vertical wells in the naturally fractured reservoirs of the Spraberry Trend Area, West Texas. Application of the model to Daqing horizontal wells indicated that these wells should have 4 to 5 times higher oil production rates if the formation was not damaged The use of the model has captured the characteristics of rapid decline in productivity of the Spraberry vertical wells. Comparisons between the effects of matrix skin and fracture permeability indicated that stress sensitive fracture permeability is responsible for the productivity loss of these wells. This paper provides reservoir engineers with a practical tool for analyzing inflow performance of wells intersecting long fractures.

Introduction

The Spraberry Trend Area of West Texas was discovered in 1949 and was considered the largest field in the world. The Spraberry Trend Area presents unusual problems for both primary production and waterflooding. After more than 40 years of waterflooding, the current oil recovery is still less than 15 per cent. A model study of a waterflood pilot in the Spraberry Trend Area indicated a NE-SW trend of the major Fracturesl. A contrast of 144/1 was required for the major/minor fracture trend permeability ratio to match the pilot response. This strong anisotropic effective permeability implies the existence of well inter-connected, long natural fractures in the Spraberry reservoir. A characteristic of flow in the long natural fractures is the pressure variation along the fracture should be significantly higher compared to that in a hydraulic fracture or a short natural fracture. Unfortunately, a method for analyzing flow behavior in reservoirs with long fractures is not readily available from the literature.

Several analytical solutions have been presented for transient flow in fractured reservoirs.2–8 Numerical models have also been developed for simulating fluid flow in fractured reservoirs.9,10 However, it is still desirable for reservoir engineers to use steady flow equations for identification of formation damage in the fractured reservoirs. This is not only because the analytical transient flow solutions and numerical simulators are not convenient to use, but also because steady or pseudo-steady flow prevails as the dominating flow mechanism in the lifetime of most oil wells. Therefore, steady flow equations are attractive for formation damage analysis.

This paper demonstrates applications of a newly developed steady-flow mathematical model for formation damage identification. This model is utilized for analyzing performance of two horizontal wells with hydraulically induced fractures in the Daqing field, China, and twelve vertical wells intersecting natural fractures in the Spraberry Trend Area, West Texas. Use of the model for matching production data aided in understanding of the productivity of the Daqing horizontal wells and the unusual behavior of Spraberry Trend Area reservoirs. The results of analysis for the Daqing horizontal wells indicate that these wells should have higher potential if formation damage could be removed. The use of the model has captured the characteristics of rapid decline in productivity of Spraberry vertical wells.

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