A New Method for Predicting Acid Penetration Distance
Authors: Roberts L.D. , GUIN J.A. 
Published: December 2007

A new method for calculating acid penetration distance in fractures bas been developed and tested experimentally. The method combines spending-time data from rotating-disk reaction pots with mass-transfer data obtained from laboratory fractures, thus allowing for both the effects of surface reaction kinetics and actual mixing patterns in the fracture. It is shown that the new method successfully predicts the acid spending obtained in laboratory fractures in both turbulent and laminar flow, using a reaction-rate constant obtained with a rotating-disk apparatus, This appears to be the first method that is easily applicable to small core samples and it allows properly for acid mixing in the fracture.

Introduction

Recently, there has been considerable interest in and research toward developing a more accurate method for calculating the acid penetration distance in a reservoir fracture. The acid penetration distance, defined as the distance the acid will travel before spending to some predetermined degree, is essential for estimating the production improvement obtainable by fracture acidizing.

The first and probably most widely used method for calculating the penetration distance was based on the static-reaction test, in which a small core sample and a known quantity of acid were allowed to react for a given time in a small pot. By equating the spending time of acid in the pot to the residence time of acid flowing down the fracture (t = L/vi), a penetration distance was calculated. it has become penetration distance was calculated. it has become apparent that, because of the extremely fast surface reaction occurring in many acid-rock systems, the over-all acid spending rate to a large degree depends on the extent of fluid mixing at the rock surface. Since fluid-mixing patterns in small reaction pots may not be necessarily the same as those occuring in fractures, several experiments have been performed using actual laboratory model fractures. performed using actual laboratory model fractures. Recent investigations of this type have shown that, because of variable fluid properties, the mixing patterns in real fractures are very complex. To patterns in real fractures are very complex. To allow for this mixing and thereby to calculate more accurately the penetration distance in real fractures, design methods based on experiments in laboratory model fractures have been developed.

Therefore, there appear to be two basic approaches to calculating the acid penetration distance, one using data obtained in small reaction pots, and the other using data gathered from model laboratory fractures. Both methods have some advantages. The former is quick, simple to operate, and applicable to the small core samples usually available for tests, while the latter method is more costly, more time consuming, requires special equipment, and is not applicable for use with small core samples. However, for reasons noted above, the latter method is probably more representative of mixing in actual reservoir fractures.

In this paper we present a new method for calculating acid penetration distance that combines the advantages of both the above methods without incurring the disadvantages. The new method combines data from both reaction-pot experiments and laboratory-model fracture tests in a manner such that both the reaction rate of the actual rock (obtained conveniently from a small core sample) and the mixing occurring in an actual fracture are allowed for. Reaction-rate constants are obtained using a small batch reaction pot containing a rotating-disk core sample. These rate constants are then used with mass-transfer coefficients obtained from laboratory fractures to predict the acid penetration distance. penetration distance.The combined mass-transfer coefficient/ rate-constant method proposed here has several advantages over existing methods for predicting penetration distance. Since general correlations penetration distance. Since general correlations can be developed for mass-transfer coefficients (in fact, many applicable correlations already exist, most notably in the related field of heat transfer), A is not necessary, nor is it usually possible, to perform experiments in laboratory fractures for each perform experiments in laboratory fractures for each new field core sample obtained.

SPEJ

P. 277