Sizing Gelant Treatments in Hydraulically Fractured Production Wells
Authors: Seright R.S. , Seldal Mailin , Liang J. 
Published: December 2007

Often, when production wells are stimulated by hydraulic fracturing, the fracture unintentionally extends through shale barriers into water zones, causing substantially increased water production. Gelant treatments have frequently been applied in an attempt to correct this problem. However, the design of the gelant volumes for these applications has been strictly empirical, and consequently, the success rates for these treatments have been erratic. In this paper, we develop a sound engineering basis for sizing gelant treatments. We present a simple 11-step procedure for sizing gelant treatments in hydraulically fractured production wells. We incorporated this procedure in user-friendly graphical-user-interface software.

Introduction

A large number of gel treatments have been applied in production wells with the objective of reducing water production without sacrificing hydrocarbon production.1 The most successful treatments occurred when the excess water production was caused either by flow behind pipe or by channeling or "coning" through fractures. For gel treatments in fractured production wells, the design of the gelant volumes has been strictly empirical. A survey of field activity revealed that the vast majority of gel treatments were very small - less than 1,000 bbl/well. The sizing of gelant treatments varies somewhat from vendor to vendor. For some vendors, the gelant volume is initially planned as 1/2 to 1 day's production volume. Other vendors plan for a certain number of barrels of gelant per foot of net pay. Still others plan to inject gelant to reach a certain radius from the wellbore The latter plan seems ironic since most treated wells are thought to be fractured, where the flow geometry is described better as linear rather than radial.

Substantial improvements are needed in the design methods used for sizing gel treatments. We strongly suspect that the most effective design procedures will vary with the type of problem being treated. In particular, different design procedures should be used for (1) flow-behind-pipe problems, (2) unfractured wells where crossflow cannot occur, (3) unfractured wells where crossflow can occur, (4) hydraulically fractured wells, and (5) naturally fractured reservoirs. The focus in this paper is on sizing gelant treatments in hydraulically fractured production wells. First, the volume of gelant that leaks off into porous rock is shown to be usually substantially greater than that in a fracture. Second, conditions are quantified when leakoff occurs at a rate that is independent of length along the fracture. Third, we quantify oil and water productivity losses and improvement in the water/oil ratio after a gel treatment. Next, parameters are discussed that are necessary to design a gel treatment, and the most expedient methods to obtain that information are identified. Finally, we present a simple 11-step procedure for sizing gelant treatments in hydraulically fractured production wells. This procedure has been incorporated in a software package.

Fracture Volume versus Leakoff Volume

When a gelant is injected, what fraction of the gelant volume locates in the fracture versus in the porous rock? Usually, the volume associated with a given fracture is quite small unless the fracture is exceptionally wide. To illustrate this point, consider a vertical two-wing fracture with height, hf, effective width, wf, porosity,, and half-length, Lf. The total fracture volume, Vf, in both wings of the fracture is given by Eq. 1.

(1)

For gelant that leaks off evenly from the fracture faces, Eq. 2 describes the relation between gelant volume in the matrix, Vm, leakoff distance, Lp, and matrix porosity, for two wings of a fracture that cut through a single zone of height, hf.

(2)

Dividing Eq. 2 by Eq. 1 reveals that the ratio, Vm/Vf, equals (2Lp)/(wf).

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