Fracturing and Production Characteristics of Tight Gas Reservoir: A Case Study from Thrace Basin, Turkey

Abstract

Abstract During the last three years, Turkish Petroleum Corporation, state owned oil company of Turkish Republic, has put significant efforts to develop its shaly sandstone reservoirs located in the Thrace Basin by-utilizing hydraulic fracturing technology. In this paper, a comprehensive review of operational and modelling aspects in the development of tight gas reservoirs were presented as a real field case study. Fracturing an over-pressured tight gas reservoir deeper than 3,000 meters (true vertical depth), in a tectonically active area, was the main challenge of the project. The initial priority was performing fracturing operations in a safe, proper, environmentally friendly way. In the post-frac phase, substantial difference observed in the individual well performances and post-frac assessment of estimated ultimate recovery (EUR) have become the main criteria in evaluation of the success of the current stimulation campaign as well as the identification of possible potential infill opportunities. Diagnostic fracture injection tests showed high closure stresses (up to 1 psi/ft gradient), which resulted in high treating pressures. To prevent screen-out, proppant concentration was increased by 0.25 ppg at each step and considerable sweeps between main stages were carried out. Different type of proppants were employed during the treatments in order to ensure having better fracture initiation and conductivity. The created fracture properties estimated from stimulation software as well as the values from rate-transient analysis were not well aligned. EUR predictions from the decline curve analysis and history matched simulation model only agreed for the first well, after having 2 years of production history, but not for other more recent wells. Image logs were also very helpful to explain different initial flow rates seen in the wells, due to the inferred presence of natural fractures. This case study summarises the efforts to overcome challenging hydraulic fracturing conditions and to analyse varying production behaviours between the hydraulically fractured wells by means of different methods; to assist the efficient development of the company's tight gas reservoirs in the Thrace Basin, Turkey. It was observed that natural fracture intensity increases as the well locations approach major faults, at which point the complex stress regime may then begin to expose the fracturing operations to a risk of early screen-out. Therefore, any proposed infill wells should be located in those areas at a safe proximity from faults for enhanced production.