Case Histories of Successful Acid Stimulation of Carbonate Completed With Horizontal Openhole Wellbores

Abstract Horizontal openhole wellbores in carbonate formations have become an everyday occurrence to improve production economics. Carbonate formations are typically stimulated by using hydrochloric acid. An acid treatment in the well configuration described provides a significant challenge in diversion to accomplish complete stimulation of the lateral. Various techniques have been employed to overcome this problem. These include pre-perforated or slotted liners, specialty treating fluids, solid diverting materials and tools. All have had some successes in various regions of the world. Presented are three case histories in which openhole packers and sliding sleeves have been employed with and without the aid of specialty treating fluids. These cases cover Devonian (TVD 10,400'), Devonian (TVD 8,800') and San Andres (TVD 5,700') formations. Details of these treatments as well as production responses are discussed. Introduction Stimulation of carbonate reservoirs is typically used to restore or enhance production to an economic level. Acid, whether organic or inorganic in nature is a natural means of effecting such stimulation of these types of lithologies1-8. Acid fracturing is the most widely used technique for deep stimulation of limestone or dolomite formations. Matrix treatments whether at a minor injection rate (low permeability reservoirs) or at high rates (high permeability reservoirs) are also commonly used to effect a shallow damage bypass. San Andres and Devonian formations are common carbonate reservoirs producing in the Permian Basin with lithologies, depths and bottomhole temperatures that vary significantly. In addition, horizontal completions are becoming routine in some areas where these formations are being produced. Geologic and Reservoir Descriptions Devonian Microscopic examination of core samples in the area of the treated wells revealed a mixture of the following lithologies: chert, cherty limestone, limey chert, dolomitic cherty limestone and dolomitic limey chert9-11. Micro porosity rather than interparticle porosity has been created by partial substitution of the original limestone matrix by silica (chert). The postdepositional processes included karst weathering, cave wall deposition and collapsed cave material accumulating as brecciated chert zones. Fig. 1 illustrates the variance of the composition through the Devonian in the wells at 8,800 ft TVD. The Devonian is a solution-gas drive reservoir with some water production. Original bottomhole pressure was found to be approximately 5600 psig from a DST in 1986. Bottomhole temperature is approximately 135 °F. Permeability varied from less than 0.01 md to greater than 16 md with an average of approximately 2 md. Porosity varies from less than 1% to greater than 13 % with an average of approximately 11 %. San Andres The San Andres (∼5,700') is a Dolomitic formation with solution gas drive in combination with gas cap expansion12-13. Average permeability is over 9 md with an average porosity greater than 13%. Acid solubility varies from 78 to 92% in 15% hydrochloric acid. The main components of the lithology are Dolomite (77 to 92%) and Anhydrite (3 to 20%). Bottomhole temperature is typically around 100 °F. These values are illustrated in Fig. 2. Devonian Case History (TVD 10,400') Over the last three years several re-completions of vertical Devonian wells to horizontal producers have been made.