Fracturing/pressure pumping

Approach Optimizes Post-Fracturing Cleanup and Testing

This paper presents an optimization project that was implemented in the Khazzan and Ghazeer fields of Oman to optimize the post-fracturing cleanup and testing period to reduce hydrocarbon flaring and CO2 emissions as well as the testing cost without compromising the overall well cleanup and testing objectives.

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The oil and gas industry currently faces the dual challenge of meeting the global energy demand with minimal carbon footprint. The Barik formation is a tight to low-end conventional reservoir in the Khazzan/Ghazeer field in central Oman. The field requires hydraulic fracturing to produce the wells economically. Each well requires a period of cleanup after pumping the chemicals and proppant into the formation for fracturing and before connecting it to the facility in order to keep all of the undesirable material from entering the processing facility. During this cleanup and testing period, all produced gas and condensate are flared in the atmosphere.

This paper presents an optimization project that was implemented in the Khazzan and Ghazeer fields to optimize the post-fracturing cleanup and testing period to reduce hydrocarbon flaring and CO2 emissions as well as the testing cost, without compromising the overall well cleanup and testing objectives.

The optimization project began in 2015, when the clean-up and testing period was reduced gradually by optimizing the well beanup schedule (choke management and duration for each choke).

Flowback of wells following fracture stimulation is an integral component in development of unconventional and low-end conventional formations. The activity has been subject to increasing interest, particularly with the North American shale revolution where multistage hydraulic fracture treatments can result in millions of gallons of water and numerous chemicals injected into the formation. Many operators are managing drawdown pressures during flowback to avoid formation damage and fracture conductivity degradation that can be caused by aggressive flowback practices. Managing drawdown pressure can maximize long-term value but requires a relatively lengthier flowback duration compared with more-aggressive practices. Extending flowback and testing periods typically requires additional capital expenditures, but perhaps a more severe consequence is greenhouse-gas emission associated with flaring. While several operators have transitioned to “green” flowbacks, many international regions, particularly during exploration and early greenfield development, do not have sufficient infrastructure in place to flow hydrocarbons to a production facility.

Khazzan is a gas field discovered in the 1990s in the Sultanate of Oman. It consists of three tight to low-end conventional reservoirs of Precambrian to Ordovician age. These high-pressure, high-temperature reservoirs include the Barik, Miqrat, and Amin, buried between 4,000 to 5,000 m true vertical depth subsea. Development of these reservoirs was initially impeded by low production rates and economic reasons until BP acquired a concession in Block 61. Following a 7-year appraisal campaign that included both vertical and horizontal wells, BP began full-field development in 2014. Later in 2016, with approval of the Sultanate of Oman, BP extended the Khazzan concession at the southern boundary, known as the Ghazeer field.

The primary development option in Block 61 is the Barik formation. Conventional, massive hydraulic fracturing is required to stimulate the Barik interval effectively. Barik post-fracturing flowback practices have evolved over time, but the fundamental strategy remains to perform a managed beanup during the flowback period. The flowback phase includes two subphases: fracture cleanup and well testing. Specific objectives of these phases follow.

Fracture cleanup includes removal of contaminants (proppant, fracturing fluids, and hydrogen sulphide) to

  • Meet permanent pipeline and facility specifications to enable safe, reliable production without compromising integrity
  • Maximize regained formation permeability and fracture conductivity relative to gas

Because the permanent production facility has a stringent operating envelope to maintain integrity and the majority of the wells will be equipped with multiphase flowmeters or venturi meters only, the well-testing phase is imperative to gain an in-depth understanding of the well capability and reservoir fluids. The specific objectives of Barik well testing are

  • Establish reservoir and well performance to support full-field infrastructure development and production planning
  • Determine producing water/gas ratio and initial condensate/gas ratio
  • Determine proppant-free gas rate at which the well can be reliably produced to permanent production facilities

This paper outlines the Barik flowback progressive strategy, which underwent significant optimization primarily through increasing data acquisition during well testing and surveillance. The primary goals of this optimization were to safely reduce greenhouse-gas emissions caused by flaring hydrocarbon and minimize the capital expenditure associated with temporary well-test packages. These goals must be achieved without compromising safety, reservoir performance, subsurface understanding, or effective fracture conductivity. Production delivery from the Khazzan field to sales was initiated in September 2017, roughly 10 years after the beginning of the appraisal campaign. Thus, performing green flowbacks was not possible until permanent production infrastructure had been built and commissioned.
Download the complete paper from SPE’s Health, Safety, Environment, and Sustainability Technical Discipline page for free until 1 June.

Find paper SPE 200179 on OnePetro here.