RIASSUNTO
Abstract
Gravel packing, being one of the most reliable and robust downhole sand control techniques, is often the preferred method for establishing sand control. In reservoirs where the pressure has fallen below the water gradient, gravel placement using water based fluid will produce a high overbalance leading to excessive losses of fluid that might deeply invade the formation, cause premature bridging, and fracture the formation. The main focus of this paper is to identify fluids and techniques for gravel packing a depleted reservoir and evaluate the best technique and a fluids package for a candidate well.
A well candidate in the Mediterranean is planned to be completed with cased hole gravel pack and it is a twin of a cased and perforated gas well that started producing formation sand after 4 years of production. Continued production from the reservoir is expected to drive down the pressure to 7.5ppg by the time, when candidate well is completed.
Several techniques for gravel packing a depleted reservoir have been considered which included diesel, alcohol-, oil-, gas- and water-based fluids. However after extensive lab testing the most suitable fluids package for the well was found to be conventional water packing. Contrary to expectation, formation damage tests indicated that water based gravel packing caused less damage than non-viscous oil based fluids for this particular candidate well. Solids-free high-temperature perforation kill pill and breaker package were also designed for the well to be stable for up to 4 days, which pushed the limits of solids free kill pills currently available in the industry.
The paper will discuss in detail the gravel packing fluid package selection methodology, and completion fluid, gravel pack fluid, kill pill and breaker fluids package design and evaluation for the candidate well.
Introduction
When completing reservoirs in weak and sanding-prone sands, oil and gas industry can offer a wealth of knowledge and years of experience and success stories. Depending on reservoir characteristics and field experience, various completion types may be appropriate including both open-hole and cased-hole sand control options. However sand control completion is always technically challenging and expensive choice. Occasioanlly wells drilled into the sanding-prone reservoirs are completed with no downhole sand control capabilities due to a variety of reasons such as inaccurate prediction of formation strength, unavailability of sand control equipment or expertise and cost or time constraints. Over time, however, sand production from the wells becomes unacceptable and the value of downhole sand control is then realized. Delayed intervention may cause the reservoir pressure to fall below the water gradient.
When reservoir characteristics require gravel pack completion, this indeed becomes the most technically challenging completion choice. Nowdays however gravel packing of pressurized reservoirs is widely used in the industry and upon vigorous design and immaculate execution generally yield good results. On the other hand, gravel packing of depleted reservoirs is not that numerous - engineering applications are rearer and each successful case story is an engineering and technological achievement. Further on in this study gravel packing fluid package selection methodology and fluids design work for well candidate in the Mediterranean will be discussed.
Well-2 is a twin gas well of the cased and perforated Well-1, that started producing formation sand after 4 years of production. The reservoir pressure is currently 5,800psi at a depth of 4,311m TVD ORKB, which equates to a pore pressure of 7.9ppg. Continued production from the reservoir is expected to drive down the pressure to 5,500psi (7.5ppg) by the time when Well-2 is planned to be drilled and completed.