The success of a stimulation technique is often measured by its stimulation ratio. This paper, however, presents a novel way of calculating the value that can be added from acid fracturing. A methodology predicting the effect of acid fracturing in carbonate reservoirs has been developed. This accounts for fracture geometry and for acid fracturing job specifications.
Acid fracturing is typically conducted in carbonate reservoirs, which make up approximately 70% of the worldwide hydrocarbon reserves. Acid fracturing is a complex process depending on multiple factors which are different in nature, making mathematical models unreliable. Due to the stochastic process inherent in acid fracturing, modeling has been met with difficulty, especially in calculating conductivity. The modeling data analysis includes the sensitivity study with respect to fracture length and fracture width, as well as the importance of efficient acid transport.
The post-stimulation production data is used to calculate the cumulative production parameter which is the cumulative production over a given period of time. It can also be compared to the production of an un-stimulated case. The cumulative production parameter offers a unique way to calculate acid fracture value. This parameter gives a tangible value which translates to production over time. More importantly, the proposed model allows fractures from different reservoirs to be accurately compared, helping production engineers make better decisions and estimate the acid treatment efficiency.
Acidized fracturing is an stimulation technique for oil and gas fields with the aim of increasing the production from a well. An acid is pumped at a pressure above the parting pressure of the rock such that the formation is hydraulically fractured. The acid reacts within the fracture to create a differentially etched surface that will maintain a conductive pathway. Carbonate reservoirs are good candidates for acid fracturing since strong acids such as hydrochloric acid react easily with carbonate.
Acid fracturing was first witnessed in 1895[1]. The Standard Oil Company was using hydrochloric acid to stimulate oil wells in carbonate formations in Ohio, USA. By comparison, the first description and observation of hydraulic fracturing was in 1935. By the 1970s propped fracturing was far more advanced than acid fracturing. During the 1970s and 1980s advancements were made in acid fracturing stimulation theory. As a result fracture acidizing has increased in popularity from the 1980s onwards[2].
Growing energy demand and the need for cost effective fracturing has lead to the widespread use of various fracture optimisation techniques. Current practice in most acid fracturing stimulation jobs is to use Net Present Value (NPV) as the main optimisation objective[3, 4]. NPV is used by systematically varying the treatment parameters and observing the change in NPV and fracture length. Such a procedure is tedious and does not take into account what treatment parameters have the most influence on the fracture geometry. However, a systematic and integrated procedure can assist the treatment design engineer in performing the design task efficiently and enforces a favourable hydraulic fracture geometry that meets various design objectives.
