Does Rock Really Weaken During Acid Fracturing Operations?
- E.O. Joel (U. of Oklahoma) | Maysam Pournik (U. of Oklahoma)
- Document ID
- Society of Petroleum Engineers
- SPE Production and Operations Symposium, 27-29 March, Oklahoma City, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 3.2.4 Acidising, 3 Production and Well Operations, 4.1.2 Separation and Treating, 4.3.4 Scale, 1.8 Formation Damage
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Acid fracturing is a stimulation process in which the dissolution of the fracture faces creates conductivity after fracture closure. The resulting fracture conductivity depends highly on the amount and pattern of etching in addition to the strength of asperities. While there has been significant amount of studies on enhancing fluid systems to generate more optimum etching pattern, etched width, and etched length, there has been minimal work on understanding how the acid systems influence rock strength and affect closure of fracture under stress.
A systematic review of all available data on rock strength used in acid fracture modeling was conducted with emphasis on determining errors involved in each measurement. Furthermore, a detailed review of documented effect of acid on rock strength was also undertaken to understand how acid changes fracture face strength. Finally, the impact of errors in rock strength values used in conductivity correlations was studied in order to determine the importance of using correct values for rock strength.
Based on analysis of measured rock strength measurements, an average of 20% has been observed in data with greater errors for dolomite formations. Furthermore, there are great variations among rock strength values among different studies, with variations up to six times. In terms of effect of acid on rock strength, the measured weakening of rock of an average of 15% is within the measurement error limits and hence no conclusion can be made on acid weakening rock strength. Effect of errors in rock strength values used in conductivity correlations suggests significant errors in conductivity estimation, especially for softer rocks and higher closure stresses.
There have been many developments in enhancing the performance of acid fractured wells. However one major reason for the lack of success is limited understanding of the effect of acid on rock strength and incorrect estimation of rock strength. In order to understand the effect of acid fracturing on rock strength and resulting fracture conductivity under closure stress, a better method of measuring rock strength is needed in addition to detailed study on the effect of treatment parameters including acid type and contact time on rock strength variation.
Acid fracturing is a common well stimulation practice used to enhance the performance of carbonate reservoirs. The process involves pumping an acid fluid at high rate and pressure to create and/or extend a fracture in the formation. The acid injected into the facture reacts with the formation to create uneven fracture surfaces which is the basis for the fracture conductivity. The importance of rock strength in maintaining sufficient fracture conductivity has been emphasized in previous studies (Greenwood & Williamson 1966; Walsh 1981; Gong 1997; Abass et al. 2006; Jaeger et al. 2007). After the acid treatment, closure stress is attempting to close the fracture, while the strength of fracture face asperities is holding the fracture open. Contact ratio, asperity distribution, and mechanical properties of the asperities are three important parameters that control the change in fracture width with closure stress. Abass et al. (2006) expressed that the asperities act as support for the conduits created from the etching process. They also highlighted how mechanical properties and processes (compressive strength, creep, and elastic response) determine the closure of the conduit created. The strength of the rock and the asperities created by the acid fracturing process determine the stress-strain behavior under closure stress. Under closure stress conditions, the elastic response and compressive strength of the rock work to keep the effective aparture open and retain fracture conductivity. The different strength properties detailed by Gangi (1978), Walsh (1981), Gong (1997), and Abass et al. (2006) in their various models attempt to describe how the strength of the rock in addition to distribution of asperities interact in keeping the fracture open under closure stress.
Acid fracture conductivity has been modeled using different approaches. There have been theoretical models and experimental correlations used to model the behavior of acid fracture under closure stress. The models attempt to directly or indirectly determine some parameters that can be used to express the behavior of fracture conductivity under closure stress. Although not all approaches were designed with acid fracture conductivity in mind, it is apparent that rock strength is a very important factor.
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