One of the major challenges of Logging-While-Drilling (LWD) Magnetic Resonance data acquisition is its limited logging speed. Typically, LWD Magnetic Resonance is logged at speeds of approximately 20m/hr (65ft/hr). Higher logging speeds will substantially reduce the vertical resolution of the data and prevent full polarization of the Hydrogen Protons in the formation, thus, introducing errors in the measurement of total porosity, fluid fractions, and permeability.
The axial motion (rate of penetration) of the magnetic resonance data has multiple effects on the acquired data. The main effects occur to the data during the polarization time, and the amplitude of the echoes during the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence time. The effects on the amplitude are broadly referred to as the flow effects.
During the drilling of a well in the Niger Delta, an operator needed to save rig cost by increasing the Rate of Penetration (ROP) for LWD Magnetic Resonance from 20m/hr to 40m/hr. This increased ROP caused an overestimation of the total porosity from magnetic resonance.
A newly introduced correction technique enables the compensation of effects due to high ROP on magnetic resonance data. This ROP correction methodology compensates flow and polarization effects. Total porosity and fluid fractions were corrected, resulting also in an updated Magnetic Resonance permeability index. Validation of the technique was accomplished by an accurate match of the corrected total porosity to results from offset wells.
This paper demonstrates the effect of a high rate of penetration on acquired LWD data. Details of flow and polarization effects, and the procedure for correcting these effects, making the data useable and accurate, are also presented.
The Magnetic Resonance (MR) method is based on a magnetic interaction of the magnetic moments of nuclei and externally applied magnetic fields. In the field of Magnetic Resonance Well Logging, only the hydrogen nuclei are of interest. Hydrocarbon and water contain a large number of hyrodrogen nuclei. The hydrogen nuclei possess the strongest magnetic moment. The hydrogen nucleus is a proton. The proton has a mass, an angular momentum and a charge. A spinning charge creates a magnetic moment. This magnetic moment allows the interaction with magnetic fields.
Due to a high noise-to-signal ratio, 10 of the 24 acquired magnetic resonance echo data logs from a field in the Niger Delta were excluded from the processing result. The reduction in the total number of echoes led to a high diffusivity, especially in the gas-bearing reservoir sand. High diffusivity caused some of the movable fluids to appear as irreducible bound water. The low hydrogen index of the gas also caused a low-permeability profile.
The effect of high diffusivity was very prominent in the Apparent Transverse Relaxation Time (T2app) spectrum, but the intrinsic transverse relaxation time (T2intrinsic) spectrum was unaffected. Consequently the, T2intrinsic spectrum was used to determine partial porosities and fractional fluid volumes. However, the total and effective porosities were not seriously impacted.
2D NMR (T2intrinsic - Diffusivity) maps were used to determine the partial porosities and fractional fluid volumes. These maps are produced by plotting diffusivity, D, against the Transverse Relaxation Time, T2. 2D NMR maps aid the discrimination of the magnetic resonance fractional volumes - clay-bound water (CBW), irreducible bound water (BVI) and movable volume (BVM). The maps can further be used for fluid-typing analysis.
The common processing result using T2app showed little or no permeability and little or no BVM in the gas interval, while the 2D NMR result showed an average permeability index of 3mD and an average movable fluid volume of 8pu. There was also very little BVI in this gas interval.
This paper effectively demonstrates the ability to use advanced magnetic resonance techniques to address issues related to high a noise-to-signal ratio, as well as minimal sensor failures. As presented, the paper clearly shows that we can get useable data from as high as 50% echo trains failure rate.
Though efforts are made to prevent signal failures and high noise-to-signal situations, there is a fix to such problems when they arise. Rather than re-log, the data can be savaged and the desired result achieved.
Gas identification and determination of Gas-Oil Contact (GOC) in reservoirs containing gas and oil can be a major challenge in laminated sand-shale sequences, where the presence of shales drastically affects the response of gamma ray, resistivity, density and neutron logs. Due to the resolution of these measurements, it becomes increasingly difficult to identify and quantify the gas reservoirs.
In a West Africa offshore well in a Cretaceous formation, using a Penta-Combo Bore Hole Assembly (BHA) with basic Formation Evaluation (FE) measurements, the use of additional services such as Nuclear Magnetic Resonance (NMR) and Formation Pressure Tester Logging While Drilling (LWD) services, significantly improved the confidence in interpretation of the reservoir fluids.
In the example well, though the size of the density-neutron crossover showed a reduction in the oil zone as compared to the gas zone to a certain degree, the actual position of the Gas/Oil contact and the reservoir fluid saturation were not certain. Using the traditional NMR porosity undercall in gas zones as well as the dual wait time (DTW) tranverse relaxation time (T2) distribution analysis, the gas zone was confirmed and the saturation of each of the fluids in the reservoir was accurately determined.
The NMR tool was programmed to acquire data in dual wait time (DTW) mode to calculate hydrocarbon saturation. The Magnetic Resonance Dual Wait Time (DTW) approach takes advantage of Longitudinal Relaxation Time (T1) contrast to solve for hydrocarbon saturation. "In light hydrocarbons, in a water-wetting reservoir, the hydrogen atoms in the hydrocarbon fluid relax slower than the nonmovable and movable water. By using two polarization or wait times (Tw), it is possible to calculate hydrocarbon saturation using magnetic resonance tools" (
Also, the gas hydrogen index effect was evident in the total porosity computation from NMR measurement. Significantly lower porosity was observed in the gas zone as compared to the oil zone. This was the first indication of Gas-Oil contact (GOC). Further analysis of the dual wait time T2 distribution gave a proper estimate of the saturation of the fluids in the reservoir.
Kim, Yonghwee (Baker Hughes) | Boyle, Keith (Chevron) | Chace, David (Baker Hughes) | Akagbosu, Pius (Baker Hughes) | Oyegwa, Akomeno (Chevron) | Wyatt, Dennis (Chevron) | Okowi, Victor (Baker Hughes) | Gade, Sandeep (Baker Hughes)
Copyright 2017, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. Annual Logging Symposium held in Oklahoma City, Oklahoma, USA, June 17-21, 2017. ABSTRACT Monitoring fluid saturations in a producing reservoir over time is critical for the effective exploitation of the resources. This can be complex in a two-phase system and is exacerbated when changes in the gas cap due to depletion or contraction, due to re-pressurization from water injection, have to be considered. Consequently, a three-phase reservoir fluid saturation measurement is crucial in determining the future of the reservoir and if remedial actions must be taken for overall optimization of the reservoir's production. To answer this challenge an advanced salinityindependent method that combines the carbon/ oxygen (C/O) analysis with gamma ray ratio-based gas saturation techniques has been developed to deliver three-phase fluid saturations. In this new method, C/O analysis and a gamma ray ratio-based gas saturation method are incorporated using an innovative triangulation technique to simultaneously quantify water, oil and gas saturations. Well-specific Monte Carlo N-Particle (MCNP)-based forward modeling enables pre-job sensitivity analysis and provides the predicted theoretical measurement responses required for log quality checks and formation evaluation in data postprocessing. This paper describes a collaborative effort by an operator and a service company to evaluate the new three-phase formation fluid saturation analysis technique to obtain post-production fluid contact and saturation in a mature field in Nigeria that was put on production in the early 1970s. In this particular well-logging campaign, the objective was to estimate the current hydrocarbon saturation. Because the formation water salinity of the subject field was low (typically less than 15,000 ppm NaCl equivalent), C/O and inelastic gamma ray ratio 1 measurements were acquired. The new triangulation method was used to integrate these measurements to provide salinity-independent three-phase fluid saturations. Results from example wells analyzed using the new technique are presented.
The Niger delta sedimentary basin is a depositional complex of Cenozoic-aged sand and shales that extend from an approximate of longitude 3° to 9° east and latitude 4° 30' to 5° 20' north. This delta is characterized by progradation, rapid sedimentation, continual loading of sediments and gravity-driven syn-depositional deformations. Hydrocarbon exploration in the Niger delta started in 1937, mainly onshore. Exploration and production now extends offshore.
Given the enormous resources that go into drilling a well, the objective is to get it right the first time (especially when drilling high-angle and horizontal wells with their associated problems of true vertical depth uncertainty and resolution of surface seismic). In drainhole sections, geological uncertainty and production technology pushes geo-steering to the limits. Sometimes, these challenges put forward questions such as: is the reservoir faulted, compartmentalized, thin, undulating or show vertical lithological changes; how far from the roof must the trajectory be; what is the minimum required drain length; and how much dogleg is acceptable?
To achieve success in the reservoir navigation of any well, some success factors must be considered: the drilling strategy, available downhole tool (drilling system and formation evaluation), surface software, personnel and communication protocol. This paper examines these success factors using the example of Well-X. The goal is to bring more understanding to the procedures involved in reservoir navigation, the challenges posed by geology, the factors to consider when planning a modern geo-steering job, the importance of teamwork, the benefits of integrated interpretation and the value communication brings to the entire process.
Ndokwu, Chidi (Baker Hughes) | Okowi, Victor (Baker Hughes) | Foekema, Nico (Baker Hughes) | Caudroit, Jerome (Addax Petroleum Development) | Jefford, Leigh (Addax Petroleum Development) | Otevwe, Joseph (Addax Petroleum Development) | Fang, Xiaodong (Addax Petroleum Development) | Idris, Maaji (Addax Petroleum Development)
High-angle or horizontal wells pose many geological challenges that include maintaining well trajectory within a particular horizon in drain sections, detecting stratigraphic positions after passing a discontinuity, and subsurface feature identification. Geo-steering has shown increased value over the years because it uses data from different sources, including borehole imaging, to meet these challenges. Bulk density and gamma ray borehole images can be used to describe the near-wellbore environment, and that description can be analyzed further to explain the near-wellbore structural geology. In this study, structural analysis and zonation of bulk density and gamma ray images were used to detect the fault zone, while a geo-steering application was used to pick the true stratigraphic depth after crossing the fault. Provision of an alternative model to seismic-only interpretations and a better understanding of subsurface structures are the industrial benefits of this study. The Niger delta sedimentary basin of Southern Nigeria is a prograding depositional complex of Cenozoic-aged sand and shales that extends from about longitude 3 - 9 E and latitude 4 30' - 5 20' N. This paper demonstrates the importance of geo-steering, shows the application of geo-steering in a high-angle well drilled in the Niger delta sedimentary basin, and establishes the importance of structural analysis from borehole images in making final geo-steering interpretations. This paper also shows that borehole imaging is an additional and useful source of information in the planning stage of any drilling campaign.