A downhole well tractor system was used in Kern County, California, USA, to convey production logging sensors into a horizontal uncemented slotted liner to determine the production profile and later coiled tubing was used to place an annular chemical packer in an effort to shut off water production.
The siliceous shale reservoirs of the Monterey formation of Kern County are very different from conventional sandstone and carbonate reservoirs. Production is primarily from thinly laminated porcelanite. One of the fundamental problems that hinder exploitation of the shale reservoirs is accurate identification of hydrocarbon and water resources. Production is achieved through extremely small pores and enhanced through small- to large-scale natural fractures. Drilling and completion techniques include extended-reach horizontal uncemented liner configurations. Several wells have experienced large water cut increases shortly after completion.
A flow profile achieved using tractor-assisted production logging sensors showed that approximately 60% of the water was produced from the bottom third of the well. An experimental technique to isolate water production in two uncemented slotted liners was employed with the use of a thixotropic annular chemical packer squeezed via coiled tubing into the slotted liner and slotted liner openhole annulus. The objective of the treatment was to shut off the bottom third of the well to reduce water rates.
Several other siliceous shale wells have been profiled, and opportunities for water isolation have been identified. Successful remediations will support the economic viability and longevity of horizontal uncemented liner completions.
Historical Attempt to Profile Production
For solid cemented and perforated completions isolated interval testing proved the most effective method of locating changing fluid contacts and profiling well production. This technique although relatively cheap and simple lacks the ability to mimic actual production mechanisms. It is well understood that inflow performance needs to be treated as a total system. Interval testing breaks the system into nodes that will not sum up to the system when commingled. This is particularly prevalent in high liquid wells, and wells being produced with gas lift.
Other attempts have included the use of a power fluid employed with concentric tubing. The technique consisted of traversing the horizontal well producing section and time and depth matching the flow on surface with the downhole intake. This technique lacked accuracy due to measuring flow rates based on flowing conditions that did not match the actual production scenario and challenges with power and produced fluid segregation on the surface.
Slotted uncemented slotted liners preclude the use of isolated interval testing. Concentric power fluids attempts lacked the accuracy and resolution required for proper reservoir management. Therefore the decision to run a comprehensive, integrated production logging tool capable of quantifying flow profile in uncemented slotted liners was chosen.
Production Log Candidate Selection
Economically based candidate selection revolved around primarily selecting wells that had relatively stable increased water cuts with the potential for water shut off. These increased water cuts invariably resulted in a reduction of oil and gas rates. Secondary objectives of the survey included understanding the production mechanism, i.e. was production well dispersed throughout the horizontal section or was it localized in the toe or heel of the well, what was the contribution from matrix versus fractures, assessing the competence of the formation behind the slotted liners, and determining the effect, if any, resulting from dolomitic tight often fractured streaks in the interval.
