The Occurrence and Control of Acid-Induced Asphaltene Sludge

Abstract

Abstract Successful acid stimulation programs are achieved when more near well-bore damage is removed than is induced by the acid treating fluid. Acid induced asphaltene sludging is becoming an increasing cause of oil well stimulation treatment failure. laboratory tests on 231 crude oils from 17 formations in the U.S. and Canada, have been tested for sludge sensitivity. Numerous chemical processes involved in an acid system contribute to the effect of acid on sludge potential. Surface tension has been used in the past as a guideline for predicting the sludge potential of a chemical system. However, surface tension, in and of itself, is not a factor in acid induced sludge. In addition, this data indicates that as oilfields mature and are placed on secondary and tertiary recovery, sludge sensitivity increases. As a result of this data, a crude oil classification system is presented based on API gravity and asphaltene content. By presented based on API gravity and asphaltene content. By classifying the crude oil, conclusions can be reached regarding sludging potential. This study addresses many such chemical options which include acid strength, acid type, solvent preflush, iron control and additive compatibility. The oxidation potential of iron contaminated acid is also clarified. Introduction An extensive study was conducted by Gidley in 1985 concluding that acid stimulation results on oil wells appear to be site specific and, therefore, appear to be more influenced by the specific nature of reservoir fluid. The fluid interaction between acid and crude oil can produce two major damage mechanisms which affect the success of acid stimulation of oil wells. These mechanisms include the formation of rigid film emulsions and the precipitation of acid induced asphaltene sludgs. While the primary focus of this paper is asphaltene sludging, rigid film emulsions are also discussed. A laboratory study of 231 crudes from 17 different formations in Canada, Texas, Alaska, California, offshore Louisiana and Mississippi was conducted (see Table 1). This study indicates that the sludge sensitivity of a crude can be statistically classified based on commonly available chemical and physical data. In addition, this study shows that commonly utilized acid systems must be adjusted in order to reduce nudging. Lowering acid strength, substituting organic acid, utilizing a solvent tireflush, controlling iron and optimizing the use of additive packages were found to reduce this potential formation damage. Scope of Statistical Study A review of the literature indicates that extensive work has been done on acid induced asphaltene sludging. However, previous studies focused on either specific crudes in limited previous studies focused on either specific crudes in limited geographic areas or on a narrow aspect of chemical effects on sludging. This study encompassed 231 crude samples from the 17 predominant formations in North America known for sludging. The authors have attempted to evaluate the effects of the entire chemical process. It is hoped that future designers of acid treatments can gain insight into a variety of chemical options which will:contribute to the achievement of stimulation objectives andminimize the occurrence of acid induced sludge. Occurrence of Asphaltene Sludge Crude oils are typically categorized as either paraffinic or asphaltenic depending on the nature of the predominant heavy species. Asphaltenes are contained in crude oils in the form of a colloidal dispersion. Amorphous in structure, the asphaltene micelle consists of high molecular weight compounds surrounded and peptized by lower weight neutral resins and aromatic hydrocarbons. The micelle consists of sheets of polycyclic rings containing 6-14 rings per sheet. These sheets polycyclic rings containing 6-14 rings per sheet. These sheets are stacked 5 deep to form the asphaltene particle. The particle has a diameter of 30 to 65 angstroms and a molecular weight ranging from 1,000 to 50,000. Asphaltene sludge which occurs when crude oil contacts acid, can be differentiated from naturally occurring asphaltene deposits. P. 99