The Integrated Characterization Techniques on Reservoir Fractures
Authors: Song Xinmin , Shen Pingping , Yuan Shiyi , Cao Hong 
Published: December 2007

Through the exploration and development practice of many fractured reservoirs, the new methods of fractured reservoir characterization have been developed. Outcrop fracture characterization is the best way of understanding subsurface fracture macrostructure, there mainly are two types of fractures: regional and structural fractures. Core fracture observation and description is the basic research of fracture analysis, and fracture aperture and its geophysical response are very important. It is the economic and effective way to identify subsurface fracture intervals by using conventional well logs, which build close tie between geology and seismic. Coherency, seismic attributes, factor exclusion and probability method based on geological modelling are integrated to predict fracture spatial distribution. The essence of geophysically detecting fracture lies in breaking free from convention and making clear "interference information". These characterization methods were applied to fracture reservoirs, and made a success.

Introduction

Reservoir fracture characterization has been the hot topic in the research of oil and gas exploration and development. For the latest decades many distinguished Schoolars, Such as Aguilera[1], Lorenz[2], Kullander[3], Nelson[4], Lynn[5], etc., has long worked at natural fracture research and achieved in natural fracture, and play a positive and promotive role in the effective development of fractured reservoirs. But natural fracture research has been in experimental study because of complicated fracture distribution. Generally, natural fracture reservoirs have high productivity, for example, the oil and gas productivity of the fractured carbonate reservoirs in the Mid-East reaches thousands of tons, even ten thousand tons. Thus, many oil companys pay great attention to the reservoir. The companys, Such as Schlumberger, Atlas, Halleburton, etc, developed many new techniques as image well logs and S-wave seismic[6][7]. Image well logs are widely used in many oil companys. Although shear wave are considered to be more reliable indicators of fracture orientation than P-wave, considerable interest remains in use of P-wave for determining fracture orientation and identification, since these form the basis of most commercial seismic survey, Therefore, we try to make full use of conventional well logging and P-wave data, combining with production test and dynamic data, to characterize fracture under the guide of geological theory, including outcrop fracture characterization, core fracture description and characterization, fracture characterization of conventional well logs and image logs, fracture characterization of special stress modelling, etc. The paper mainly discuss the integrated characterization of outcrop, core, log and seismic methods.

Outcrop Fracture Characterization

Outcrop fracture observation is very important in understanding 3-D fracture orientation, the relation to regional structure and lithologies. Based on a lot of outcrop fracture investigation, two types of fractures are apparently observed, i.e. regional and structural fractures.

Regional fracture distributes in the wide extent with geological regularities, and relates to regional structural movement, even plate movement, and has no origine relation to local fold and fault. It plays a instructive role in regional oil and gas development. Figure 1 represents the outcrop fracture of sand conglomerate reservoir in the Northwest China. The fracture distributes in the wide range in the Zhungeer basin, especially in the western part of the basin. The reservoir belongs to the lower permian, under which the regional fracture is found in the Carboniferous slate. The reservoir lithology is mainly conglomerate, wide fracture spacing occurs in middle- coarse conglomerate and narrow fracture spacing occurs in middle-fine conglomerate. In the adjoining subsurface reservoir, the regional fracture well developed. So, the outcrop fracture study was applied to the adjoing subsurface reservoir development, the later production data support outcrop investigation.