A Practical and Economical Fracturing Solution For Low Permeability Shallow Reservoirs
Authors: Gu F. , Kuru E. , Zhao Z. , Mo Z. 
Published: March 2010

Fluid cost saving is critical for fracturing operations in low permeability reservoirs where the production revenues are low but the job size is relatively large and the fluid cost is high. Cross-linked fluids (CLF) are usually the first option. However, they may cause significant damage to both propped fractures and formations, and are not the cheapest option. Polymer-free fluids, on the other hand, cause much less damage but they are expensive and fluid costs may impair the economic results of fracturing. Waterfrac would be a compromise solution for low permeability reservoirs since its fluids are cheap and fluid damage is low. The success of waterfrac with low slurry concentrations is, however, difficult to predict.

This paper presents a new fluid system that was formulated to maximize the economic return of fracturing wells in low permeability shallow oil reservoirs. It is a solid-free, linear, synthetic polymer-based system with a very low formation damage characteristic. The new fluid system can meet a variety of fracturing requirements, including slurry concentrations of conventional field levels. Moreover, it is much cheaper than cross-linked guar gel (CLGG).

The method for designing fluid components and the procedure for preparing the fluid to achieve minimum formation damage and minimize the cost are described. A comparison of the production performances from the same well or adjacent reference wells fractured with the new fluids and CLGG is made. The reservoir geology, fluid type, and operation data of fracturing, and well performances from more than 300 successful wells in three different low permeability shallow oil reservoirs (800 ∼ 1,500 m depth) are also presented in detail.

Introduction

Hydraulic fracturing is a necessary technique to develop low permeability reservoirs. Fracturing fluid takes a paramount role in fracturing treatments. In most cases, cross-linked fluids (CLF) can meet the treatment requirements. However, severe formation damage frequently occurred when cross-linked guar gel (CLGG) was used. The core flow tests by Devine et al.(1) showed that the permeability reduction by CLGG was 56% ∼ 71% for cores of 100 ∼ 200 mD, and 12% ∼ 35% for cores of less than 1 mD. The fluid damage to the tight sand was less severe than that of a permeable formation(1, 2). The experiment study by Almond(3) at 120 ° F with 20/40 mesh sand packs (simulating fractures) showed that the flow reduction by CLGG damage could be as high as 100%. Without doubt, damage of this order of magnitude will greatly decrease the productivity of fractured wells, whether the damage happens in formations or in propped fractures. Therefore, fluid impairment by CLGG could be a significant problem adversely influencing the success of fracturing operations.

Fluid cost is another critical factor of fracturing, especially in low permeability and tight reservoirs where the job size is large and the fluid cost is high, but the well productivity is low and declines faster than that of fractured wells in conventional reservoirs. Fluid cost saving is a key factor to economically develop these reservoirs, in particular when they are marginal reservoirs.