Stimulation should always begin with a good understanding of the well history and the reservoir rock. This should include an evaluation of the formation composition and flow characteristics, permeability and porosity, and the mechanical properties of the rock. Determination of the "Key Factors" limiting stimulation success should be identified. The potential treatment fluids and technique should be evaluated for the ability to provide the necessary benefits to overcome these limits.
This paper describes stimulation design and results in a carbonate gas field located in eastern Saudi Arabia. The field produces gas with H2S that varies from 0 to 10 vol%. The Khuff reservoirs, at 11,000 to 12,000 ft, are highly acid soluble carbonates (> 85 wt%) with elevated bottom hole temperatures approaching 275°F and a composition having increasing dolomitization with increasing depth. Acid leak-off control problems result from the presence of natural fractures in the formation. Various retarded acid systems were tested with reservoir rocks. In addition, the issues of corrosion control were considered given the low-carbon steel tubulars downhole (L-80 &C-95) and the H2S. Although the gas is sour, proper design of acid systems eliminates problems of iron sulfide and elemental sulfur precipitation.
Evaluation of cored material showed 15 wt% anhydrite streaks. Kinetics studies conducted using reservoir rock indicated that the reaction rate of the in-situ gelled acid is an order of magnitude lower than that of neat acid. An acceptable corrosion rate on all metals by the acid in the presence of H2S and compatibility of all systems with reservoir fluids were obtained. Wells have been treated using acid fracturing techniques employing viscous fluids without encountering any operational problems. Substantial increases in gas production and flowing wellhead pressures have resulted from these treatments. Analysis of well flow back samples collected following acid fracture treatments was used to assess the effectiveness of the fracture treatments.
Introduction
Stimulation of carbonate reservoirs is typically the result of a need for restoration or enhancement of production to a more economic level. The design of these treatments requires a great deal of input to be successful. To begin with a thorough understanding of the reservoir is essential. This should include complete knowledge of the composition of both the mineralogy and the fluids, flow characteristics, permeability, porosity, and mechanical properties. After this is the need for a good knowledge of the well's history. Complete records of the drilling operations, production history, workovers, changes in wellbore parameters and the wellbore configuration. Lastly, as accurate a diagnosis of the damage or restriction inhibiting the economic production level. From all of this information will come the "Key's" to what to use and how to conduct the treatment.
Certain wells by their nature add additional constraints or hurdles to overcome to reach the desired production increases. One of these is depth and therefore higher temperature and possible treating rate limitations. The higher temperature increases corrosion rate of well tubulars and limits depth of penetration due to increased reactivity, if acids are to be used as the treating fluids. Another well type, which adds additional concerns, is one that produces hydrogen sulfide (H2S) gas. The major effects on design are to deal with the potential of additional corrosion and the damaging reaction products possible from treating acids and the H2S.
Recently, Saudi Aramco embarked on an extensive campaign to develop its gas fields. Acid fracturing has been used to enhance the performance of gas wells in a tight carbonate gas reservoir. In-situ gelled acids were extensively used in these treatments. These acids consist of a synthetic polymer, a crosslinker, and a breaker, and other acid additives. They cross-linker over a narrow pH range where their viscosity increase by several order of magnitude. The objectives of this work were to:evaluate in-situ gelled acids to stimulate deep carbonate gas reservoirs,assess the treatment in the field, andoptimize the treatment based on lab and field results.
