Redevelopment of an Unconventional Oil Reservoir: Finding Unstimulated and Bypassed Reservoir Volume
Authors: Fayzullin Maksim , Janiczek Peter , Contreras David , Makar Ivan , Gonzalez Jose , Day Richard 
Published: November 2017

This paper identifies re-stimulation opportunities in existing horizontal wells with existing multistage hydraulic fracturing to increase oil production in a tight carbonate formation, offshore Black Sea. Pilot candidates were screened and ranked through the following decision criteria described in this paper, coupling reservoir, production and completion parameters. Favorable pilot candidates were verified by numerical simulation, which also highlighted potential areas of by-passed oil for future sidetracks.

In an era of low oil prices, it is increasingly difficult to justify drilling new wells, especially in the offshore environment. A strategic shift towards revamping and workovers has made operators of tight and unconventional reservoirs to focus on restimulation. Many factors define success herein, and the key to finding the right candidates. Synergy between numerous parameters, combined often in indexes or drivers by their nature, is used to score and prioritize existing well potential and associated risks. On the other hand, reservoir and its understanding still play a major role and prevail over solely statistical methods. Top scoring refracturing candidates in this work were simulated in both full field and sector models. Real post-job fracture geometries and newly initiated fractures were critical inputs into unstructured reservoir simulation grids to ensure that the identified targets of the restimulation pilot wells are realistic and achievable.

Recent multistage stimulation jobs in this field accidentally led to several "frac hits" (cross-well communication initiated while pumping a hydraulic fracturing treatment), which were confirmed by tracer analysis in the adjacent wells. Subsequent offset well behavior had both positive and negative effects, thus enabling the quantification of gains from restimulation, where possible, and intrinsic well interference. The choice between refracturing old or new wells, and finding the balance were the major pitfalls in this work. Wells, drilled 5-7 years ago and stimulated with typically 3 stages, had sub-optimal completions for refracturing, however were placed in the better quality rock, therefore displayed higher initial production. Delineating the reservoir bodies from seismic inversion and integrating with the simulation model, highlighted favorable well placements and historically it was proven to reduce initial water cut by 2-3 times. Recent and more extended wells were positioned in the tighter, "saddle" zones, drilled and completed couple of years ago. They had due drawdown management, flatter decline, plus less mechanical and operational risks. Pay in these extended wells was recognized as a major uncertainty when history matching flowing bottomhole pressures.

Results of the statistical scoring and decision criteria, to enable selection of refracturing candidates, are presented in this work, along with the integration and coupling these outcomes with the applied reservoir simulation. Both sets of results were used to find new opportunities for the mature field redevelopment.