Abstract

Matrix acidization has long been an important completion and stimulation tool. This paper discusses underlying concepts, treatment techniques, and results of acidizing both carbonates and sandstones.

Introduction

Acidization has been a useful process in initiating and promoting petroleum production for many years. Without completing a history of the process, a summary of developments are in order to bring us to the present day. Originally, acid was dumped or bailed into place and allowed to sit until it had done its work. Development of high pressure pumps allowed displacement of acid to formation and slow squeezes into the formation. Encouraged by the success of hydrochloric on carbonate formations, researchers tried hydrofluoric acid on sandstone reservoirs, but until recently HF had not been as successful as predicted or hoped. Recent research has concentrated on two areas: sandstone acidizing, its techniques problems criteria 4,5,6; and with acid fracturing, the parameters, design, and prediction of results 1,2,3. In the leap of technology to acid fracturing, matrix treatment of carbonates has been severely overlooked. The purpose of the presentpaper is to bring together discussions of matrix carbonate acidizing, with summaries of sandstone acidizing, post treatmentevaluation and examples.

Any discussion of matrix processes should be prefaced by a definition of the adjective matrix. We have confined the meaning to those processes which take place within the pores of the rock and which act on that portion of the rock surrounding the pores. Although fracturing affects the rock matrix (by cleavage and dissolution), it is not considered a matrix process. We, therefore, believe that the important part of the definition is the confinement to pore spaces.

The large increases predicted for and expected from fracturing have tended to eliminate matrix acidizing other than as a prelude to fracture treatments. There are, however, many other applications for matrix treatments which will increase productive capacity and efficiency.

Situations in which to consider matrix acidizing are:

  1. presence of nearby gas cap or acquifer;

  2. surface or well constraints preventing fracturing;

  3. allowable or productive limitations on investment recovery,

  4. poor fracturing success;

  5. cost constraints on treatment performed.

In the main, these are self explanatory, but a bit of elaboration on item (4) is in order. There are many cases where matrix acidizing can be successful even though fracturing has failed. Sandstones soft enough, that all flow capacity is lost through embedment, or hard enough that proppant has crushed, blocking flow channels; or carbonates that etch so uniformly or leave such weak insolubles that the fracture closes on release of treating pressure will respond to matrix techniques.

Treatment Mechanics

It is very important that the treatment be confined to the pore spaces. Particularly this is true in the initial stages of the treatment.

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