Abstract

This paper presents the results and interpretation for return permeability testing following the leak-off of fracturing fluid in reservoir rock samples containing movable oil saturation. The dynamic fluid loss tests were conducted with 2 inch diameter and 10 inch long core samples. The fluids used were linear guar and hydroxypropyl guar (HPG), and borate crosslinked guar and HPG, respectively. Tests were conducted with and without fluid loss additive (FLA) in order to study its effect on return permeability impairment. All tests were conducted at room temperature. The drive pressure during leak-off was 500 psi.

Following the leak-off, the cell was shut-in for a duration of at least 3 hours. In order to obtain the variation of return permeability with time and distance from the fracture face, the return permeability was determined for each 2 inch long section of the core sample. Variation of return permeability for the specified range of fracturing fluids is presented and discussed.

Introduction

Hydraulic fracturing is a widely used stimulation technique for optimizing the productivity of a well. Until the early 80's, only low permeability (k < 10 ind) gas bearing formations were routinely hydraulically fractured. However, recently there has been a tremendous interest in fracturing higher permeability formations, commonly referred to as "Frac-Pack". Some of the important criteria for a successful "Frac-Pack" treatment are:

  1. Formation damage due to fluid leak-off should be minimum.

  2. Proppant should be uniformly distributed in the fracture.

  3. The impairment to fracture conductivity should be low.

In this paper we are focusing on the permeability impairment caused by the fracturing fluid leak-off in high permeability formations and its subsequent recovery.

During fracturing, some of the fracturing fluid leaks off into the formation. Significantly large leak-off in high permeability formations may have the potential of reducing the permeability near the fracture face, thereby seriously affecting the well producibility. The leak-off can be controlled with the aid of fluid loss additives. But, the addition of fluid loss additives may also severely reduce the fracture face permeability.

The impairment of formation permeability caused by fracturing fluid leak-off in presence of 100% brine saturation has been addressed in the past. However, according to Gadiyar et. al. leak-off in the presence of mobile oil saturation is significantly different compared to leak-off in 100% brine saturated cores. Therefore, the return permeability and its recovery after leak-off in oil cores is expected to be different from that observed in 100% brine saturated cores.

The objective of this study reported is to evaluate and characterize the extent of impairment in formation permeability to oil, due to the leak-off of fracturing fluid in oil reservoirs. This included the investigation of the variation of permeability with distance from the fracture face and its subsequent recovery as a function of production from the reservoir.

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