Optimization of Hydraulic Fracturing in a Deep, Multilayered, Gas-Condensate Reservoir

Abstract

Abstract Two deep, gas condensate reservoirs are under development in Oman to supply a peak capacity of 40 million m3/d gas for Liquefied Natural Gas (LNG) export and domestic customers. To date (May 2000) 37 multiply-fractured development wells, with a total of 106 hydraulic fracture treatments have been completed in the multi-layered, heterogeneous Barik and Saih Rawl fields. A continuous optimization process was in place from the beginning to capture all aspects of the project's learning and reduce development cost. The frac design changes over time as a result of this process will be discussed. Initial well capacity of the wells cleaned up and tested to date is 20% higher than planned, offering considerable project savings. This paper will describe the way in which the significant challenges in fracturing these deep, multi-layered gas-condensate reservoirs were met. Difficulties in interpreting and analyzing FETs (Fluid Efficiency Tests) caused by varying stress contrasts in the different layers will be discussed. It will be shown that masking early data of the pressure decline after shutdown in the minifrac can result in misinterpretation of leakoff parameters. Well tests performed at regular intervals gave a better understanding of the optimized relationship between length, width, and height, which in turn led to a revised fracturing design. Perforating strategies influencing the placement of individual fracturing treatments, in particular affecting fracture connected height, will be addressed. Modeling reservoir characteristic has presented significant hurdles with existing fracture simulators, primarily caused by the amount of detail required to accurately predict both the fracturing and productivity behavior in the heterogeneous Barik and Saih Rawl fields. Introduction General Overview The Oman LNG project is initially developing the deep gas-condensate fields of Saih Rawl and Barik in Central Oman. Petroleum Development Oman acts as upstream operator for the 100% Government owned gas resources. The project has a peak gas processing capacity of 40 million m3/d of gas and 12,500 m3/d of condensate. To ensure the initial gas supply some 37 wells were drilled and fractured from the start of the development phase, at the end of 1997. There is a potential future well requirement of 40 to 60 over the life of the fields. Field Development Plan (FDP) A number of exploration and appraisal wells were drilled in both fields during the early and mid 1990s, in which various completions were evaluated, including perforated wells, vertical wells with both small and large scale hydraulic fractures, and one multiple fractured horizontal well. Those trials pointed into the direction of a multiple fractured vertical well as the most efficient development option for both fields. Main uncertainties at the time of the FDP (1996) were the range of production improvements that could be generated by the hydraulic fracturing process and medium-to-long term production behavior, because of reservoir permeability, coupled to condensate drop-out in the reservoir. For this reason the plan proposed an initial development campaign, designed to provide the required initial production capacity for the LNG plant and a buffer of about one year's production decline. During this buffer period, well numbers for the second development phase could be adjusted upwards or downwards if required, to ensure the gas supply would be secure once the 25 year LNG contracts were running. General Overview The Oman LNG project is initially developing the deep gas-condensate fields of Saih Rawl and Barik in Central Oman. Petroleum Development Oman acts as upstream operator for the 100% Government owned gas resources. The project has a peak gas processing capacity of 40 million m3/d of gas and 12,500 m3/d of condensate. To ensure the initial gas supply some 37 wells were drilled and fractured from the start of the development phase, at the end of 1997. There is a potential future well requirement of 40 to 60 over the life of the fields. Field Development Plan (FDP) A number of exploration and appraisal wells were drilled in both fields during the early and mid 1990s, in which various completions were evaluated, including perforated wells, vertical wells with both small and large scale hydraulic fractures, and one multiple fractured horizontal well. Those trials pointed into the direction of a multiple fractured vertical well as the most efficient development option for both fields. Main uncertainties at the time of the FDP (1996) were the range of production improvements that could be generated by the hydraulic fracturing process and medium-to-long term production behavior, because of reservoir permeability, coupled to condensate drop-out in the reservoir. For this reason the plan proposed an initial development campaign, designed to provide the required initial production capacity for the LNG plant and a buffer of about one year's production decline. During this buffer period, well numbers for the second development phase could be adjusted upwards or downwards if required, to ensure the gas supply would be secure once the 25 year LNG contracts were running.