New Subsea Safety Tree for Extreme Environments Successfully Meets Challenges in Brazil for Drill-Stem Testing/Fracturing Jobs
Authors: Marcancola Fernando , Nolasco Hugo , Barcelos Rita , Rubio Luis 
Published: June 2014

Deep-water exploration in Brazil has continued to increase in complexity, requiring new technology to not only meet the ever increasing demands of the challenging environments but also the increase in costs. These have been incurred from the continuous increase and upgrades in safety regulations and the costs associated with exploration into new environments. In this scenario, operators are welcoming any cost-effective methods to evaluate well profitability without sacrificing safety.

In a newly discovered potential oil block, it is necessary to obtain downhole data such as pressure, temperature under specific conditions (flow and build-up periods) as well as obtain pressure-compensated fluid samples and produced volume across time. To obtain these data, it is necessary to perform Drill-Stem Tests (DSTs). Using the type of data that can be derived from DSTs, it is possible to estimate the volume of the reservoir, its layers inside the tested field, and other characteristics that are necessary for planning completion. If performed offshore, testing requires safety equipment placed inside the blowout preventer (BOP) stack to keep the well under control, prevent undesired flow, and protect the environment and personnel.

Recent discoveries in Brazil indicate that there is a large hydrocarbon potential in the pre-salt area. That area extends in a track that includes 800 kilometers from the northeast to the southern regions, is 200 kilometers wide away from the coast, and has reservoirs that are 3,000 meters deep. The initial estimation of hydrocarbon production for the Brazilian pre-salt area was approximately 60 billion barrels, but other research has shown different results that are estimated to be from 120 to 200 billion barrels. The importance of this information has more than justified the need to perform DST operations to obtain reliable data.

According to Petrobras, Brazilian production has increased steadily, and production has been 300 thousand barrels/day since 2008, when pre-salt production in Brazil commenced; the company wants to reach 1 million barrels/day by 2017. To achieve those numbers, it will be necessary to guarantee that the reservoir will respond as originally predicted. This will require acquisition of reliable data to estimate the current resources, which the DST provides. After the DST, if the operator determines that the production from the formation has not performed according to their expectation or their desired production rate, then the operator may choose to fracture the well. This process is performed by injecting large amounts of a specific, proppant-laden fluid into the reservoir at a high pressures and high pump rates, in order to attempt to increase the reservoir's expected production.

With the traditionally used equipment, after completing the fracturing process, the equipment must be pulled out of the hole (POOH) and tripped in again to perform the actual drill-stem testing operation. Several trips were required, because the safety equipment available for the oil industry was not certified to work in extreme environments with solids being pumped at high rates and pressures. If the trips were to be consolidated, a specially-designed downhole equipment package would have to be developed.

A major engineering/service company has now developed new subsea safety-tree equipment to be placed inside the BOP stack; with this new equipment, the fracturing operation and the drill-stem testing can be performed in the same trip, since the safety valve system has the capability to maintain integrity when functioning in heavy proppant, high pressure, and high pump-rate conditions.