Case Histories of Successful Acid Stimulation of Carbonate Completed With Horizontal Open Hole Wellbores
Authors: Metcalf A.S. , Orona S. , Kretzschmer G. 
Published: March 2010

Abstract
Horizontal open hole 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 open hole packers and sliding sleeves have been employed with and without the aid of specialty treating fluids. These cases cover Devonian TVD 3,170 m (10,400 ft), Devonian TVD 2,682 m (8,800 ft) and San Andres TVD 1,737 m (5,700 ft) 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 lithologies(1–8). 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 chert(9–11). Microporosity, rather than interparticle porosity, has been created by partial substitution of the original limestone matrix by silica (chert). The post-depositional processes included karst weathering, cave wall deposition and collapsed cave material accumulating as brecciated chert zones. Figure 1 illustrates the variance of the composition through the Devonian in the wells at 2,682 m (8,800 ft) TVD.
The Devonian is a solution gas drive reservoir with some water production. Original bottomhole pressure was found to be approximately 38,611 kPa (5,600 psig) from a DST in 1986. Bottomhole temperature is approximately 57 °C (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 (~1,737 m, 5,700 ft) is a Dolomitic Formation with solution gas drive in combination with gas cap expansion(9, 12). 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 38 °C (100 °F). These values are illustrated in Figure 2.