A New Strategy Explores Tight Oil/Gas Reservoirs: Fit-for-Purpose Acid Fracturing
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164778, ’A New Strategy To Explore Tight Oil/Gas Reservoirs: Fit-for- Purpose Acid Fracturing,’ by G.M. Hegazy, SPE, and Adel M. Salem, American University in Cairo and Suez University; Shedid A. Shedid, British University in Egypt; Shouhdi E. Shalaby, Suez University; and J. Abbott, Schlumberger, prepared for the 2013 North Africa Technical Conference and Exhibition, Cairo, 15-17 April. The paper has not been peer reviewed.

As oil companies have moved to more-marginal reservoir targets, application of conventional techniques has often yielded disappointing results, and tighter zones are often abandoned for more-promising target intervals. Marginal reservoirs may require horizontal-well drilling or multiple-stage hydraulic fracturing to achieve economic production targets. In an effort to reduce risks and costs associated with these alternative processes, a new stimulation strategy has been adopted for tight intervals using vertical wells before going to horizontal wells.

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There are tens of undeveloped reservoirs and even undiscovered plays/prospects in the area of this study (Abu Dhabi). These new plays and undeveloped reservoirs contain huge quantities of stock-tank oil initially in place (STOIIP). Most of these reservoirs are still in the appraisal phase, and tremendous efforts are required to bring the best reservoirs to the top of the development portfolio. The decision to transfer a single reservoir from appraisal to development mostly depends on STOIIP ranking, which in turn takes into account the following factors.

Exploration-, Appraisal-, and Development-Data Investigation. The study investigates 51 reservoirs allocated with-in 14 fields under the appraisal portfolio, and three exploration reservoirs dis covered recently after applying the new concept of fit-for-purpose acid fracturing. The work is extended to include a development reservoir in which long-horizontal-well drilling is implemented while the study recommends fracturing implementation. Fig. 1 illustrates the distribution of STOIIP percentage among the three phases of exploration, appraisal, and development. The investigation of well-test results shows that more than 180 wells were tested in the area of the study. For 98 wells, the test results were considered inconclusive because of either reservoir tightness or improper stimulation treatment, or both.

Reservoir Ranking by STOIIP. On the basis of STOIIP ranking results, 13 reservoirs out of a total of 54 were ranked highly, with a minimum of 100 million STB. These reservoirs represent 77% of the total STOIIP of the portfolio. Not all of these high-STOIIP reservoirs could produce with matrix-stimulation treatment, however, because the permeability of 12 reservoirs is less than 4 md. In the class of reservoirs ranging between 100 and 50 million STB, 17 reservoirs represent some 17% of the total STOIIP of the portfolio. The remaining reservoirs are very small, with STOIIP values of less than 50 million STB.