Selective Matrix Acidizing of Horizontal Wells
Authors: Da Motta E.P. , Hill A.D. , Sepehrnoori K. 
Published: March 2010

Abstract
The productivity index of a horizontal well in an anisotropic medium is calculated during a process of partial acidization in which damage removal occurs only over a fraction of the length of the well. Two numerical simulators are used in this work: an acidizing simulator which calculates the permeability distribution around the wellbore and a reservoir simulator which calculates the productivity index of the well. By applying mathematical transformations, it was possible to reduce the acidizing model from a two-dimensional (2D) problem to a one-dimensional (lD) one.
By comparing treatment results, it is possible to, select the optimal volume of acid, injection rate, and fraction of the length to be acidized for each particular well. The same procedure may also be applied for vertical wells both with and without anisotropy.
Simulation results indicate that the application of a partial acidizing strategy reduces the total amount, of acid required for a significant improvement in well productivity. This may substantially reduce theinancial, operational, and environmental risks involved in the treatment of a horizontal well. In most cases, the optimal injection rate for sandstone acidizing is the maximum rate which does not fracture the formation.

Introduction
Matrix acidizing of vertical wells is a reasonably well understood technique and is generally modelled using a radial geometry. In recent years, however, the wide use of horizontal wells has required that the standard procedure for matrix acidizing be adapted to the new environment. Two major factors must be considered when designing an acid treatment for a horizontal well. First, the area exposed to the formation is much larger, due to the extended length of the wells. Second, the normally large contrast between the horizontal and vertical permeabilities must now be taken into account. Applying the conventional design procedure to a horizontal well would result in using huge amounts of acid and extremely long operations, with corrosion problems impossible to avoid using currently known corrosion inhibitors. It is, therefore, very important that a new technique be developed to acidize horizontal wells that reduces the total volume of acid to be used.
The main objective of this work is to use numerical simulators in order to evaluate the feasibility of partially acidizing horizontal wells. Figure 1 illustrates this concept with the arrows showing the relative flow of fluids into the well after the acidizing is completed. An acceptable productivity increase can be obtained without having to inject an excessive amount of acid if only a fraction of the well length is treated. Issues such as the optimum volume of acid and optimum injection rate are addressed. Two criteria have been proposed for the injection rate: the maximum possible(1, 2) and an optimum rate that minimizes the acid volume(3, 4). Partial acidizing means to remove formation damage only in a few equally spaced intervals of the well, while the other segments remain untreated.
FIGURE 1: Schematic of the partial acidizing concept. Illustrations available in full of paper.
FIGURE 2: Schematic of the anisotropic problem. Illustrations available in full of paper.