Relative Permeability Modifiers in Fracture Stimulation Applications
Authors: Dalrymple Eldon Dwyann , Jaripatke Omkar A. 
Published: March 2009

Abstract
The industry is expanding by hydraulic fracture stimulation of reservoirs that had previously been passed over because of low porosity, low permeability, high water saturations, and/or the targeted interval being in close proximity to water. Water production following a hydraulic fracturing operation into these types of scenarios can become a great concern, often making the stimulation of these intervals an uneconomical venture.
The addition of a hydrophobically modified, water-soluble, relative permeability modifier polymer to water-based fracturing fluids has been found to provide conformance while fracturing (CWF) treatment enhancement benefits. CWF can reduce the quantity of post-stimulation water production and increase fracturing fluid efficiency.
This paper discusses how water production has been significantly reduced using a CWF method incorporating a hydrophobically modified, water-soluble polymer (HMWSP) relative permeability modifier (RPM) into the fracturing treatment design. The purpose of the HMWSP is to reduce the effective permeability to the flow of water without significant changes to the flow of the hydrocarbon.
The increase in water production following a hydraulic fracture stimulation using the CWF method is typically less than 50% that of offset wells fracture stimulated using conventional methods. Additionally, the total production of the wells stimulated using the CWF method usually has resulted in lower water-gas ratios. This paper describes the properties of the HMWSP, general CWF job design considerations, field applications, and post-fracturing result case history comparisons of the CWF method with offset wells.

Introduction
Hydraulic fracturing is widely used to increase recovery from maturing reservoirs. Minimizing the quantity of produced water that typically accompanies increased hydrocarbon production is of the utmost importance. The traditional approach has been to limit well selection for fracturing treatments to candidates where stimulation can be efficiently performed without a subsequent increase of post-frac water cut. This significantly reduces the desired development of targeted fields.
In general, there are two main reasons for water production increase:(1) fracture propagation to water-saturated formations caused by the absence of reliable barriers and(2) phenomenon of water coning caused by pressure drawdown through a highly conductive fracture channel during the exploitation period.
To mitigate the problem of post-stimulation water production increase, a RPM can be applied. The RPM described here is a HMWSP that can be pumped as a preflush to fracturing treatments or throughout the whole job. Under formation conditions, HMWSPs decrease the relative permeability of rock (reservoir) to water with almost no effect on the permeability of hydrocarbons. This is crucial because it allows production of higher oil volumes without unacceptable quantities of associated water production.

Theorized RPM Mechanism
The governing mechanism for an RPM is segregated flow of oil and water (Fig. 1) (Torres et al. 2006). Because of pore-size restriction, an RPM fluid, placed in a porous media, will selectively restrict the flow of the phase in which it is soluble. This mechanism holds for both gel systems and single-polymer systems that demonstrate retention. The location of retained RPM fluid in the pore space is governed by wettability in the same way as the preferred pathways for water and oil. In water-wet media, the water pathways are mainly located close to the surface and in the smallest pores, where entrance pressure restricts the flow of oil. In oil-wet media, the water pathways are mainly in the middle of the largest pores and pore channels.