Acharya, M. N. (Kuwait Oil Company) | Joshi, G. (Kuwait Oil Company) | Al-Mershed, A. (Kuwait Oil Company) | Al-Otaibi, F. (Kuwait Oil Company) | Al-Azmi, Mejbel (Kuwait Oil Company) | Dashti, Q. M. (Kuwait Oil Company) | Wiryoutomo, M. D. (Schlumberger Oilfield Eastern Ltd) | Chakravorty, S. (Schlumberger Oilfield Eastern Ltd)
Is the seal leaking? Does the cap rock strata have full integrity? Is the system in equilibrium over geological time? Are there any natural flow dynamics prevalent even when the reservoirs are virgin? These are few Frequently Asked Questions (FAQs). These FAQs becomes very critical for any geologically complex and structurally challenging deep high pressured and high temperature, unconventional reservoirs. The unconventional tight-fractured carbonates and kerogen resource-play reservoirs of Deep North Kuwait Fields, underlain by anhydrite-salt layers of cap rock, have given rise to similar questions in the early stages of development for successful economic exploitation and field development.
Stoneley waveform data can be used to identify fractured interval and estimate open fracture’s width in a borehole. The method uses direct and reflected Stoneley wave arrival measured by the full waveform array sonic tool. The Stoneley wave, when passing a fracture which is intersected across a borehole, applies pressure to the fluid in the crack. If the fracture is open, some fluid will flow into it and will result in increasing pressure drop in the borehole. The magnitude of the pressure drop depends on the fracture opening and extent thus giving an indication of fracture permeability. Dropped of pressure attenuates the direct Stoneley wave and also generates reflected Stoneley wave.
The spectral noise log (SNL) logs are designed to identify different fluid flow mediums in the reservoirs, cross-flows behind casing and tubing and casing leaks by spectral analysis of recorded noise signals. Qualitative and quantitative analysis of SNL data require filtering techniques that can extract statistically spectral components, and the useful signals expected to be confined to a near well bore region of investigation. The two data sets complement each other.
The paper discusses the integrated results of a couple of wells, where the SNL data is complemented with open-hole full range Stoneley waveform data to answer some of these questions. Thus taking considerable lead for reservoir management, field development planning and effective well integrity management and surveillance plan.
Acharya, M. N. (Kuwait Oil Company) | Al-Mershed, A. (Kuwait Oil Company) | Narhari, S. R. (Kuwait Oil Company) | Al-Azmi, M. (Kuwait Oil Company) | Dashti, Q. M. (Kuwait Oil Company) | Chakravorty, S. (Schlumberger Oilfield Eastern Ltd) | Abdulkadir, R. I. (Shell Kuwait E&P) | Prosvirkin, S. (TGT Oil & Gas Services)
Kuwait Oil Company is developing deep tight fractured carbonate and kerogen rich shale gas plays in northern part of Kuwait. Understanding the flow medium is important to resolve ingress of offending fluids such as water and salt during production history in the vertical/deviated wells of this play.
These unconventional reservoirs have been established to be hydrocarbon producing in several prolific vertical producer wells, without having such early water breakthrough. The tight fractured limestone reservoir is sandwiched between salt-anhydride sequence above (Gotnia cap rock) and Kerogen rich carbonate below. A dedicated casing is set at the top of limestone reservoir with the main objective of isolating the Gotnia section prior to opening the reservoir, as some sequences of anhydrites with calcite stringers in Gotnia are high pressured and prone to high water, CO2 and H2S. Based on the current understanding, main source of offending fluids is suspected to be the overlying Gotnia formation, which has limestone stringers.
Fracture studies from several conventional cores and image log data could not conclusively infer through going fractures into Gotnia, although many sub-vertical fractures of different nature have been observed and interpreted in the reservoir section in these logs.
This paper highlights the approach for detection and characterization of flow mediums, in the near well bore region (NWR) within a diameter of investigation of 3 meters or 10 feet. Results of spectral noise log (SNL), high precision temperature (HPT), pressure and natural radioactivity (GR) of a vertical well logged in shut in and flowing mode helped in understanding different flow mediums such as channel flow, fracture flow and reservoir flow. Thus the detection and characterization of different flow path systems with respect to their spatial dimension and their interactive flow contribution could be ascertained with high confidence.
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.