Unconventionally Hydraulic Fractured Wells to Develop a Tight Sour Oil Reservoir
Authors: Mandhr Al Kalbani , Yong Ouyang , Ameera Al Harrasi , Saif Al Jarradi , Dmitrii Smirnov , Kifah Al Tobi , Aida Al Khusaibai , Tom Lefeber , Al Ghaliya Al Shabibi , Jose Viota , Zuwaida Al Saadi 
Published: September 2023

A field in the southern Sultanate of Oman producing from a tight salicylate reservoir is being studied to evaluate its development potential with unconventional hydraulically fractured vertical wells after initial results from two proof-of-concept wells were positive. The reservoir is more than 3,000 m deep, trapped in salt and over-pressured. It contains sour oil with H2S/CO2 contents of 1 and 2 mol%, respectively, and highly saline formation brine. Although the reservoir has good porosity, the formation is tight with micro-Darcy permeability. Of the eight producers present, two producers have been fracked unconventionally hydraulically with 6 and 15 stages, respectively.

The study aims to develop such a field with unconventionally hydraulically fracked producers to mature commercial volumes and meet the value drivers of the project. Development of the field is intended to be phased, starting with the crestal area to reduce risk, and progressing to the flanks over time.

The project went through several phases and milestones to explore project decisions and select the optimal options to meet the project's value drivers. The team was tasked with economically optimizing the number of wells, well placement and frac design, well material and completion, and developing a solution for halite scale precipitation.

Frac parameters were first matched in Ghofer frac model to match frac job parameters for the two existing unconventionally fracked wells. Dynamic simulation models with frac models were used to match well test and Production-Log (PLT) data. These models were then used to generate production forecasts to support project decision-making and maximize value while ensuring project competitiveness within the company's project portfolio.

To address halite scaling potential during frac fluid recovery, water samples were collected and analyzed for compatibility and scaling potential and to optimize a scaling mitigation strategy.

The two unconventionally fracked wells produced longer fractures than the conventional wells and have shown to deliver greater economic oil rates. Sonic noise log and well test results demonstrate that guar-based gel produces thicker and shorter cracks. High-viscosity friction reducer (HVFR) produces thinner but much longer fractures. Unconventional fracs increase reservoir coverage by three times compared to conventionally fracked wells, resulting in a five-fold increase in oil recovery at higher pipe-head pressures.

From these findings, the decision is made for unconventional fracs, cleaner HVFR frac fluid, and fracture conductivity damage of about 15-25%.

The optimal unconventional frac design and well spacing were considered dependent and evaluated in combination. Key well design, completion, and material decisions were made considering production conditions to safely produce commercial oil. To reduce the outcome uncertainty and implementation risk while developing the field, multiple scenario trees were constructed, which were used to decide on the phased manner for field development.

The study shows that well spacing and optimal number of wells should be studied with frac design. All decisions related to subsurface, frac technology and well design should be made in an integrated manner, considering the circumstances of the project to make optimal decisions.