The Use of Coiled Tubing During Matrix Acidizing of Carbonate Reservoirs

Abstract

A laboratory and field study directed at improved well performance of horizontal wells is discussed. During the study, several wells were matrix acidized using bullhead and coiled tubing placement techniques. The study performed in carbonate reservoirs indicates acid placed with coiled tubing diverted with foam provides excellent zone coverage and damage removal. Conventional bullhead techniques do not result in effective damage removal. The study emphasizes the evaluation of the treatment results and the development of improved acidizing techniques. Laboratory simulations of matrix acidizing indicate proper placement techniques are essential. This observation is supported by field data in oil wells completed in carbonate reservoirs. The key to successful damage removal is 1) the placement of acid via coiled tubing and 2) proper diversion. Production logging and well performance data support this claim. The proposed treatment is applicable in both horizontal and vertical wells completed in carbonate reservoirs. Introduction Stimulation of horizontal wells has increased in recent years and become a popular topic in the literature. Stimulation techniques include propped hydraulic fracturing, acid fracturing and matrix acidizing. The selection of the stimulation technique is dependent on the type of reservoir, the production objectives and economics. Matrix acidizing treatments are normally less expensive than fracturing treatments and are applicable in a significant percentage of the horizontal wells drilled today. For example, a 2000 foot horizontal well with 1 md permeability and 50 foot of zone could produce approximately 400 BOPD with skin of 20 and 1500 BOPD with zero skin and the same drawdown. Thus, matrix acidizing of horizontal and vertical wells completed in long sections with adequate kh should be considered the preferred stimulation technique followed by the more "aggressive" techniques fracturing Matrix treatments are designed to restore the natural permeability of sandstones and stimulate carbonates (create negative skin). It is preferred to hydraulic fracturing when fracturing may result in water or undesirable gas production and well performance objectives can be economically reached by removing the formation damage. The success of fracturing can be low in naturally fractured (fissured) reservoirs where horizontal drilling is popular. The poor success is attributed to propagation of the hydraulic fracture parallel to the fissure system and its inability to intersect other fissure systems. Selection of the proper placement technique is a key issue during stimulation of long intervals/zones. Today operators use both bullhead and a variety of coiled tubing placement techniques during matrix stimulation. This paper discusses the overall design methodology for coiled tubing placement with emphasis on the placement techniques including diversion during matrix acidizing of carbonate reservoirs. The recommended placement technique incorporates foam as a diverter. Laboratory and modelling studies were performed and used to develop the subject procedure. Case histories including evaluation logs are presented for wells treated using the bullhead and the recommended coiled tubing technique. Focus is on horizontal wells. P. 139

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