The Innovative Production Enhancement Strategy of using High GOR Crude in HP/HT Reservoir to Lift Upper Reservoirs’ Production, Case Study in Iraq

Abstract

Halfaya oilfield in Iraq, discovered in 1976, contains multiple vertically stacked reservoirs. Currently main oil reservoirs are under primary development. The remaining deepest high GOR, HP/HT carbonate reservoir with original pressure gradient of 0.867 psi/ft has been drilled, tested, but undeveloped. An innovative production strategy has been put forward highlighting that high GOR crude can be injected into the upper oil reservoirs with partially pressure depletion, as energy-conservation measure, in view of its high wellhead pressure and good productivity during drill stem tests, therefore to achieve production enhancement and synergy among reservoirs. In-house studies indicate high GOR crude can be miscible with all oils from eight zones. Field results show that injection processes can bring dead wells back production, yielding the prolific output. Also, water cut can be reduced immediately and field water cut rising trend can be mitigated. The innovative injection processes are illustrated as follows: New development wells should be deployed at updip positions to penetrate the all the hydrocarbon reservoirs with the thick pay. Run tubing string with packer. Packer is used for isolation of upper oil zones and HP/HT environment. Huff and puff process. After perforation, high GOR crude travels up the tubing string, via surface choke and a manifold with flow meter, then enter the annulus. Choke regulates the injection rate. For low-permeability (<0.1md) carbonate reservoir, e.g., Sadi reservoir, huff and puff processes are applied post multistage fracturing treatment. High GOR flood is preferred in majorly producing reservoirs with high productivity and relatively low GOR, e.g. Mishrif reservoir. Injected high GOR crude (API=32.7) can dilute remaining oil (API=19.1-23.0), reduce its viscosity, and therefore achieve production enhancement. During primary development, asphaltene deposit occurred and chemical inhibitor has been applied. After oil blending, the risk of asphaltene deposit could be minimized. First perforate lower-pressure oil zones and then open the HP/HT zone for work safety. The production enhancement strategy and processes in this paper could be of strong reference value to similar stacked reservoirs.