Experimental Research on Development Fracturing Technology of Changqing Ultra-Low permeability Reservoirs
Authors: Hi Wenrui , Song Zhenyun , Liu Hailang , Ren Yanpeng 
Published: December 2007

Abstract
The main producing layer is L6 reservoir of Triassic in Jingan oilfield, Changqing. It belongs to ultra-low permeability, low pressure and low production reservoir with permeability 1–2×10-3µm2, average porosity 12%. To enhance EOR, so as to reach economical and high efficiency development, several researches are made on development fracturing technics. Ideas, results and treatment effects of development fracturing technology are discussed in this paper. The main characteristic of this technology is to optimize fracturing fracture system and development well pattern by reservoir numerical simulation, as to reach the best match between them, and to realize high efficiency development.

Introduction
After researches on reservoir geology, in-situ stress and fracture orientation, lab pilot experiments of development fracturing made in 6km2 area of well ZJ60 region from 1997, Changqing ultra-low permeability oilfield is decided to model rectangle producer-injector pattern with 480×330 oil well and 960×330 injection well. By numerical simulation, fracture penetration ratio is defined primarily as 70%(propped fracture length is 170m), with a fracture conductivity of 22.5µm2.cm.
Based on lab experiment research made in1997, 34 producing wells and 15 injecting wells are drilled in this area in 1998. In addition, ten experimental researches, including fracturing performance monitoring, fracture orientation testing, pressure buildup well testing, injection well stimulation mode and so on, are made. Results show propped fracture length reaches 145.9m-172.5m. Compared with adjacent offset wells under the same condition, the effect is prominent in early stage, and the average oil production of single well is 9.15t/d, which is 1.08t/d higher than the average single offset well production. After producing for a year, the average production rate declines to 6.37t/d, which is still 1.01t/d higher than the offset wells.

Reservoir Characteristic
Well ZJ60 region is primarily river mouth bar sedimentary. Sand thickness is relatively large, good continuity. Lithology is a set of medium-fine lithic feldspath sand. Average porosity and permeability of L6 reservoir is respectively 12.24% and 1.267×10-3µm2. Pore throat sorting is poor with a low withdrawal efficiency. Pressure coefficient of producing layer is 0.618MPa/100m. Crude oil viscosity is 2.06mPa.s with a density of 0.756g/cm3. Ground water, with pH value 6, belongs to CaCl2 type.
Sensitivity analysis shows L6 reservoir has weak velocity sensitivity, medium to medium-weak acid sensitivity, medium weak water sensitivity.

In-situ Stress Orientation and Fracture Azimuth
One of the most important bases of development fracturing simulation research is the direction of in-situ stress. However, orientation of fracturing fracture is parallel to direction of the maximum principal stress. Direction of in-situ stress is obtained by coring testing and formation dip logging.
The fracture azimuth can be obtained by downhole microseism tests. During hydraulic fracturing, formation cracking generate seism signal, which can be send to three-phased sensors (one vertical sensor, V, and two horizontal sensors, H1 and H2) in monitoring well in form of wave. Then, fracture orientation can be obtained by analyzing signals written by tape handler.
(1) Use first amplitude of compressional wave received by two horizontal sensors H1 and H2 to decide orientation of seismic source.
(2) Use time difference of transversal wave and compressional wave to define the length of seismic source. If velocity of transversal wave and compressional wave are defined as Vs and Vc, time reaching to sensor as ts and tc, the distance from the seixmic source as D.