Abstrast Developed California oilfields are often under pressured. These low pressures and relatively high permeabilities present challenges in supporting mud and cement columns during well construction. Accurate formation pressures are critical for petroleum exploitation. They are used for geological correlation, drilling/completion design, surface facilities design, and for reservoir management. Obtaining formation pressures in under pressured or tight reservoirs in open hole can be challenging due to poor hole conditions, thick mud cakes, risks of getting differential cable or tool stuck, and supercharging effects. These conditions can either compromise the formation pressure data quality and or result in formation pressure tool fishing operations. This paper will show the results of a two well case study where lightweight cements of 9.8 ppg where successful run and evaluated for single stage cement jobs in a stacked reservoir with under pressured and over pressured formations. Then a cased hole formation pressure tool was deployed to get a combined total of twelve reservoir pressures after which the tool restored the casing to a condition that allowed for successful down the casing multi-stage high pressure proppant fracture jobs. This combination of new technologies for mature reservoirs has given operators more options for well design and formation evaluation. Data quality and the answers uncovered have helped justified the continuation of infill drilling opportunities to increase production and reserves. Introduction New technologies have created opportunities for operators to exploit previously uneconomic formations. Stacked or laminated reservoirs often contain formations that have been producing for decades combined with formations that have been largely undeveloped. Advances in drilling, reservoir characterization, completion, and stimulation coupled with secondary and tertiary recovery processes have moved more reserves into the producing and probable classes. These diverse formations present unique challenges for total field development. Declining pressures associated with primary depletion methods typically yield under pressured reservoirs. Normally pressured or even over pressured formations can exist in the same field and are commonly being exploited together as the envelope for what is economical expands. Accurately assessing the individual zone pressures are key for improved reservoir energy management. Cost effective drilling and completion techniques are essential for continued development. Hydraulically isolating cement sheaths are key for stacked formations where selective stimulations, water and or gas conformance, and advanced recovery processes are embedded in the business plan for the present and future. This paper will describe a similar environment where integrating technologies of light weight cements, advance cement evaluation, and cased hole derived formation pressures were used to aid in the development process. Reservoir Description Shallow, below wave base, marine shelf deposits have formed a series of fine-grained, silty, sandstones and siltstones that are highly variable in reservoir rock quality. Although the porosities are high (up to 30%) the clays have resulted in a large variation of liquid permeabilities (with a approximate range of two magnitudes, (i.e. 1 md to 100 md). Decades of production from the higher permeability sands have resulted in under pressured formations. These under pressured formations lay above the higher-pressure tighter formations. Both zone types are now being aggressively exploited with new methods of stimulation and completion technologies. (Figures 1 & 2).

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.

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