Acharya, Mihira Narayan (Kuwait Oil Company) | Kabir, Mir Md Rezaul (KOC) | Al-Ajmi, Saad Abdulrahman Hassan (Kuwait Oil Company) | Dashti, Qasem M. (Kuwait Oil Company) | Al-anzi, Ealian H.D. (Kuwait Oil Company) | Kho, Djisan (Schlumberger Pty Ltd) | Darous, Christophe (Schlumberger Oilfield Eastern Limited) | Chakravorty, Sandeep (Schlumberger Oilfield Eastern Ltd)
Flow capacity evaluation in carbonate reservoirs is known to be challenging because of heterogeneity in the rock matrix. The original depositional texture and resulting pore structure is often altered by secondary diagenetic processes such as dissolution, leaching, cementation, and dolomitization, creating complicated pore systems with varying porosity to permeability relationship. Dolomitization in particular is known to be an important diagenetic process in carbonate reservoirs, typically enhancing porosity and permeability development and making the rock less susceptible to porosity reduction due to increasing effective stress during burial. Core data taken in deep carbonate reservoirs reveal a strong correlation between degree of dolomitization and reservoir quality.
Neutron-induced gamma-ray spectroscopy logging has proven to be a powerful tool for the evaluation of dolomite content, especially in wells drilled with barite-weighted mud where PhotoElectric Factor (PEF) is not reliable. Using methods developed on a core database, reservoir rock types can be identified and matrix permeability can be estimated from a combination of porosity and dolomite content derived from neutron-induced gamma-ray spectroscopy data and other common logs measurements. Predicted flow profiles and flow capacity of the reservoirs can be calculated from the estimated matrix permeability and can be verified by comparison with available production logs and test data.
Several examples will highlight the comparison between the predicted synthetic flow profiles and the flow profiles measured by production logs, as well as the comparison of estimated flow capacity with pressure transient analysis data. Such comparisons can be used to diagnose stimulation effectiveness, identify zones dominated by fractures, confirm solid bitumen effects, and identify zones with significant formation damage. Another important application is the selection of perforation and stimulation zones to achieve optimum production based on the expected permeability contrast. This integrated approach to flow capacity prediction is proving to be an effective tool in understanding the behavior of complex carbonate reservoirs
There are two important questions that are always the sustainability foundation of any oil and gas reservoirs development. They are: a) how much hydrocarbon is present or what is the storage capacity? The answers are related to the knowledge of porosity, saturation, area, and thickness of the reservoirs. b) can it be produced economically or later, how can it best be produced to achieve the highest economy benefits? The answers are related to the flow capacity of the reservoir which is a function of permeability.
Porosity, saturation and reservoir thicknesses can generally be derived at the wells from different techniques and logging tools, such as neutron, density, sonic, resistivities and magnetic resonance. On the other hand the flow capacity evaluation which is a dynamic property is known to be challenging, especially in carbonates. Carbonate rocks are chemically unstable and prone to dissolution, leaching, cementation, dolomitization and overburden compaction. These natural processes generally occur after the original deposition creating heterogeneity in carbonate matrix and especially impacting the rock's permeability.
Solution for a Long-Standing Cementing Challenge—Intelligent Cement Set Control Additive
Industry has always found it challenging to cement long zones with a single slurry system. A huge temperature differential exists between the bottom and the top of a long cement column. Cement slurry designed for the circulating temperature at the bottom of the long column will sometimes fail to set at the low temperatures at the top of the cement column. These results in rig nonproductive time and in certain cases can lead to well integrity problems.
While setting unplanned kickoff plugs or plugging and abandoning wells, the bottom hole temperature may not be very well known. The slurry setting is sensitive to temperature, and the setting time can change substantially if the actual bottom hole temperature is different from that used for design. This leads to failure of the cement plugs. This issue is more pronounced around 200°F.
A new generation of engineered cement set control (ECSC) additive has been developed to successfully cement wells in the scenarios mentioned above. The additive works intelligently and not only provides sufficient placement time for bottom of long cement columns where temperature is higher, but at the same time allows for fast compressive strength development even at the top, where temperatures are lower.
A single slurry design incorporating the additive has been used successfully in setting multiple plugs at different depths, with varying bottom hole circulating temperatures, demonstrating the relative insensitivity of the retarder to temperature variation.
This paper discusses successful use of the additive in the field for setting multiple plugs at varying depths and temperatures using the same slurry design. Field cases with results related to the various applications will be provided.