Abstract

Field applications of combined acid and proppant fracturing have been increasingly used since originally proposed a few decades ago. To some, uniting the deep wormholing ability of acidizing with proppant transportation is a natural progression toward greatly effective stimulation of highly soluble carbonate reservoirs. However, two major obstacles remain with this process (dependent on treatment order):

  1. increased leakoff when acid is pumped first and

  2. proppant overdisplacement when proppant is pumped first.

To address these issues, this paper proposes a simultaneous acid and proppant fracturing (SAPF) strategy using an invert emulsified acid (IEA) as a dual purpose fluid to perform simultaneous stimulation.

SAPF combined with IEA offers the following advantages compared to using traditional stimulation methods:

  • Simultaneous proppant fracturing and acid wormholing.

  • Minimized formation softening and proppant embedment attributed to a truly retarded system.

  • Easy breaking back into low-viscosity phases (with only heat and time).

  • Little to no residue left in the formation (minimal or no polymer loading).

  • Can be fully premixed (no need for on-the-fly components) and stored for extended periods of time (weeks).

  • Reduced operational complexity/risks.

This stimulation technology advancement is viewed as a significant step toward developing innovative and efficient solutions to increasing needs from fields in South America, the Middle East, and Africa (particularly the pre-salt formations of Brazil and Angola).

Results from rheological and settling testing are presented, verifying the use of IEA to transport proppant and create wormholes for simultaneous matrix stimulation. The rheological properties of IEA and crosslinked gels are compared, demonstrating comparable viscosities for proppant transport and low treating pressures. Settling tests with proppants demonstrate that the formulation can be tailored to accommodate various requirements for proppant transportation.

Introduction

In reservoir stimulation, proppant fracturing uses non-reactive fracturing fluids to place the proppant inside the fracture to retain fracture conductivity after the fracture closes. As an alternative, acid fracturing is proppant-free and depends on nonuniform acid-etching on the fracture surface. This creates increased fracture conductivity only as long as the differential etching can support against fracture closure. In reality, the effective conductivity of the acid-etched fractures is often shortened by crushing of the tight fracture channels attributed to high bottomhole pressures (BHPs). For clean and homogeneous carbonates, acid fracturing creates mainly uniform etched fractures, which can close completely and have very low retained conductivity. To extend and sustain acid fracture conductivity and use the features of both proppant fracturing and acid fracturing, the two methods in combination were studied by Bale et al. (2010) for carbonate stimulation. This work verified that retarded acids were the ideal choice for the combined stimulation method. Nevertheless, neither proppant transport capability nor wormhole characteristics of retarded acids were verified with laboratory experiments.

As noted, retarded acid systems provide several advantages when attempting combined acid fracturing with proppant.

Hydrochloric (HCl), formic, and acetic acids are the main acids used in carbonate stimulation. Formic and acetic acids are naturally retarded, while hydrochloric acid becomes retarded once it is gelled or emulsified because of decreased acid diffusion rates. The diffusion rate of gelled HCl is approximately one order of magnitude lower than ungelled HCl, and the diffusion rate of emulsified acid is approximately two orders of magnitude lower than plain acid (Buijse and van Domelen 1998).

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