Optimizing Fracture Stimulations in Low-Permeability Oil Reservoirs in the Ordos Basin
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 158562, ’Optimizing Fracture Stimulations in Low-Permeability Oil Reservoirs in the Ordos Basin,’ by Xinghui Liu, SPE, Pinnacle—A Halliburton Service; Yanrong Chang, Hongjun Lu, Baochun Chen, Jianshan Li, Yin Qi, and Chengwang Wang, PetroChina Changqing Oilfield Company, prepared for the 2012 SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, 22-24 October. The paper has not been peer reviewed.

Huge low-permeability oil reserves are found in the Ordos basin in China. Key characteristics are low reservoir quality (10 to 13% porosity), low permeability (0.3 to 2.0 md), and low reservoir pressures (pressure gradients of 0.0075 to 0.009 MPa/m). Fracture stimulation is essential for commercial production in these reservoirs. To understand fracture-growth behavior better and to optimize treatment designs, microseismic monitoring and fracture modeling were performed for pilot wells in key areas. In some reservoirs in the basin, fracture-closure stress sometimes could not be obtained from commonly used pump-in/shut-in tests because pressure decline is extremely slow. To obtain closure stress in these reservoirs, conventional pump-in/flowback (PI/FB) tests were modified and used. Fracture-modeling analysis then was conducted with a calibrated fracture model, which must match both the observed net fracture pressures and the fracture dimensions from microseismic mapping. This approach led to significant improvements in fracture-treatment designs in these tight oil reservoirs.

Introduction

The Ordos basin is a lacustrine/fluvial and sedimentary basin in northern China and covers approximately 370  000 km2 across the Shaanxi, Gansu, and Shanxi provinces, and the Ningxia and Inner Mongolia autonomous regions, as shown in Fig. 1. Oil reservoirs are found at depths ranging from 800 to 2800 m in the south, and natural-gas reservoirs are found at depths ranging from 2700 to 3600 m in the north. Oil reservoirs are mainly Triassic and Jurassic systems, while natural-gas reservoirs are mainly Permian and Ordovician systems. The Ordos basin also has abundant coal and coalbed methane. Fracture stimulation of the low-permeability oil reservoirs in the Changqing oil field of the Ordos basin was studied. Changqing oil field is a general term that represents many individual oil fields across a large area.

Oil production in the Changqing oil field started in 1971. During early development, oil was produced only from reservoirs with relatively good properties (permeability of 10 to 100 md) in the Jurassic formations, and small-scale fracturing treatments were necessary to remove near-wellbore formation damage. In recent years, these Jurassic reservoirs with relatively good properties have been depleted. Currently, the Triassic reservoirs with poorer reservoir properties account for more than 85% of the proved oil reserves in the Changqing oil field. The Triassic reservoirs are characterized by low reservoir quality (10 to 13% porosity), low reservoir permeability (0.3 to 2.0 md), and low reservoir pressures (pressure gradients of 0.0075 to 0.009 MPa/m). Development of the low- permeability reservoirs in the Changqing oil field began in the 1990s, while the large-scale development of ultralow-permeability reservoirs began in 2008. Production from these reservoirs requires the use of fracture stimulation.