Multistage Frac Zonal Isolation in Extreme HPHT Conditions - Solution to Succeed
Authors: Sayapov Ernest , Nunez Alvaro , Farei Ibrahim , Benchekor Ahmed , Hazzaa Osama , Mesbahi Tarik , Thomson Matthew , Strom Kyle 
Published: November 2019

In current depressed oilfield industry environment, economical and operational effectiveness becomes even more important especially in complicated, challenging projects that demand large investments and simultaneous utilization of multiple technical services. In Petroleum Development Oman (PDO), there are a few gas fields having similar downhole conditions with multiple target pay zones, whereas fracturing operations are complicated by the requirement of CT cleanouts and/or milling in between the stages. Multizonal and multistage frac operations are commonplace in oil and gas reserves around the globe, however despite of increased number of wells stimulated using specialist multistage completion equipment, the most often utilized completion design for this operation is still plug and perf, especially in vertical wells. There are also techniques on the market involving coiled tubing for perforation and isolation between the stages, however they have their own constraints and limitations, especially in high-pressure and high-temperature environment. For PDO wells, multistage completion technologies were not feasible, therefore conventional plug & perf approach had been selected as the optimum option. The main drivers behind this selection are the challenges associated with precise deployment of the completion jewellery across small pay zones and limited coverage of the target zones when using frac sleeves. Another constraint in the past was the pressure rating of the multistage completion systems existed on the market.

Plug and perf completions are designed to allow pinpoint placement of isolation and reservoir access with on the fly adjustability. This means that there is more freedom in selecting desired perforation interval, plug-setting depth and no additional restrictions on the pumping rates that are incurred by CT string inside the tubing as in some of the popular techniques. The zonal isolation is the portion of the design that allows the frac treatment to address the target intervals without affecting the others. In operations requiring 15k+ differential pressure ratings, isolation becomes extremely challenging and requires robust and reliable technology to ensure true integrity so stimulations can be placed as per design. This challenge may get even worse with increasing temperatures, whereas conventional composite compounds are not applicable due to "swelling", or getting softer. The primary job of the frac plug is to isolate but operational safety and millability also must be taken into consideration for the overall efficiency of a completion design. Additional challenges in the target fields are the depletion of the zones and their extreme breakdown pressures that are not only exposing frac plugs to extreme differential stresses but also causing difficulties during milling operations, whereas maintaining balanced circulation becomes a primary task in order to prevent coiled tubing differentially or mechanically sticking in the wellbore. That is adding another requirement for the frac plugs - be easily millable and produce as small cuttings as possible to improve debris removal.

The high pressures, high temperatures, aggressive fluids and long exposure time seen by isolation plugs in these frac operations had been a challenge for current treatment and completion designs due to availability (and reliability) of equipment and tools represented on the market, therefore Petroleum Development Oman and the frac service provider collaborated to engineer a successful solution. It involved multiple steps, such as development of the specific design requirements, integrity-testing criteria and field trials before the plug could be adopted for wide application. The target design was a plug with a ~3.45" maximum OD, having true 15k+ differential rating at ~400F+. This composite/metallic hybrid plug was supposed to be able to withstand the harsh downhole environment seen in the various PDO fields in Oman land operations and resolve multiple complications associated with well interventions in these fields.