Members SPE-AIME Abstract Pretreatment analysis, job planning, and well preparation lead to acidizing success in sandstones with permeabilities greater than 50 md. Formation mineral analysis improves success in sandstones with lower permeabilities. Injection pressure responses to acid injection provide data for onsite decisions. Introduction Many papers have been written about specific pro ducts for acidizing sandstone formations. Most often these products are designed to correct specific problems and are primarily based on laboratory research in sandstone cores. Very little has been written about evaluating problems that exist in oil and gas wells completed in sandstone formations. Identifying problems in real wells which can be solved with specific products is an important part of the overall acidizing program. This paper will attempt to provide insights into identifying problems and selecting the best product or process to remove the specific damage. Also, many papers have been written about specific acidizing design models. Most of these papers predict the spending of HF as it penetrates the formation and sometimes the permeability increase. These papers show that regular 3 percent HF only penetrates formations about 6-12 inches before the HF completely spends. More recent retarded HF acids have achieved deeper penetration. It has been the authors experience that the volume of acid predict by these models is not the real key to successful acidizing, but rather it is the control of injection into all the perforations in the formation that determines success. Moreover, some formations respond well to HF acids and others can be damaged by the application of either HF acid or HF acid. This paper will discuss the reasons for this with recommendation on which wells are potential HF acid successes and which wells should be stimulated in another way. PLANNING THE ACID TREATMENT The first step in planning an acidizing treatment is to determine whether the well is damaged and how much. One should determine the production potential of the well to see whether removing the damage will provide enough production increase to pay for the acid treatment in a reasonable period of time. The question to be asked in evaluating well damage are:when was the well damaged,how was it damaged, andwhat caused the damage. Production history curves often show when a well was damaged unless the damage occurred during drilling and completion. it is important for the engineer to understand all aspects of formation damage in order to interpret the records that exist in well files. Formation Damage Analysis Certain types of damage consistently occur in the three major phases of a well's life:drilling and cementing,completion andproduction. Information exists or can be obtained to show whether damage could have occurred and its mechanism. Drilling and Cementing Damage One of the most important sources of damage is that from drilling mud filtrates which usually have a high pH. Several authors, have shown that pH's above 11 are damaging to formations with significant quantities of clay, i.e. 5-20% by weight of clay. A recent paper, showed the variation of permeability with pH. This curve can be used as a first estimate to determine the reduction in permeability by invasion of formations with high pH mud filtrates.
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