Abstract

During fracturing of high permeability reservoirs hydraulically there is always a concern that fracturing fluid leak-off may cause a reduction in permeability. One important criterion of a successful fracturing treatment is a limited formation damage. This study is focused on investigating the alteration of permeability with distance from the fracture face, during fracturing fluid leak-off, and its subsequent recovery during production.

The experiments are conducted in a 2 inch diameter and 12 inch long dynamic fluid loss core holder. Ten inch long berea sandstone core samples of varying permeabilities are used. Four different types of fracturing fluids are tested. Also, the effect of fluid loss additive on regain permeability is investigated. The movable oil phase in the core sample is either mineral or crude oil.

The variation of return permeability with time and distance is determined along the length of the core sample. Effects of shut-in time, fluid loss additive, fracturing fluids, flow rates during flowback, and oil composition on regain permeability of oil reservoirs is presented in this paper.

Introduction

In recent years, tremendous interest has been developed in hydraulically fracturing high permeability formations.1–3 The objective is to improve the production by creating propped fractures which aid in favorably altering the natural flow profiles. In order to optimize the productivity, the fractures created should have a very high conductivity and also the impairment in formation permeability due to fluid leak-off should be minimum. The fracturing fluids play a major role in the success of a fracturing treatment of high permeability formation. Therefore, the fracturing fluids should exhibit the following desirable properties:

  1. Cause minimum damage to formation permeability due to interaction of filtrate with the matrix

  2. Good proppant carrying capacity

  3. Minimum fracture conductivity damage

  4. Fluid loss should be minimum

In this paper, the emphasis is on the impact of fracturing fluid leak-off on the regain/return permeability of oil reservoirs.

While fracturing a reservoir hydraulically, the fracturing fluid has a tendency to leak-off into the reservoir. The fluid which leaks off into the formation is generally termed as filtrate. The filtrate consists of polymer and/or particulates which can plug the pore throats, thereby reducing the near fracture face permeability. Usually the leak-off is enormous in high permeability reservoirs, due to which there is a potential of drastically reducing the near fracture face permeability. This can lead to a reduction in productivity of the reservoir.

Very few researchers4–6 have investigated the impairment in formation permeability caused due to fracturing fluid leak-off in high permeability reservoirs. Further, all of the past studies were conducted in 100% brine saturated core samples. However, the formations that are hydraulically fractured contain movable oil and/or gas in addition to brine. It has been shown by Gadiyar et. al.7 that the presence of movable oil significantly alters the leak-off behavior of the fracturing fluids. Therefore, one would expect the return permeability and its recovery after leak-off in oil core to be different compared to that observed in 100% brine saturated core sample.

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