Rojas, Pedro A. Romero (Weatherford International) | Cristea, Alexandrina (Weatherford International) | Pavlakos, Paul (Weatherford International) | Ergündüz, Okan (ARAR AS) | Kececioglu, Tayfun (ARAR AS) | Alpay, Server Fatih (ARAR AS)
Nuclear magnetic resonance wireline logging and data post-processing technologies are continuously evolving, making significant contributions to rock, fluid typing, formation evaluation and characterization of the near-wellbore zone. In heavy oil fields, however, nuclear magnetic resonance (NMR) logging is known to provide an underestimated permeability, poor reliable oil typing and thus poor oil saturation and viscosity determinations, especially when the evaluation is based only on the spectra of transverse magnetic relaxation times (T2) (one-dimension NMR) [Romero et al., 2009]. Several attempts have been made to improve NMR results, mostly with limited success [Fang et al., 2004], especially in separating the oil component from the contribution of other fluids to the T2 spectra. The main reason lies not necessarily in the selection of the data acquisition parameters and sequences for a single-frequency or multi-frequency tool, but in the way how the data is post-processed.
The present study refers to a well drilled through the Derdere formation, a limestone/dolomite heavy oil reservoir in Turkey. The NMR data was acquired in with a centralized, single-frequency wireline tool in a 6-in. borehole, drilled with water-based mud in a freshwater carbonate reservoir. The generated T2 log was analyzed in a traditional way to obtain the NMR total porosity and its partitions based on standard cutoff values. For the given 12 API oil gravity, reservoir temperature (76 °C) and gas-oil-ratio (GOR) the T2Oil peak appears around 170 ms, right from the T2 cutoff for limestones; therefore, no corrections were needed on the permeability calculated from the Timur-Coates and Schlumberger-Doll-Research (SDR) equations. In the present well, only a diffused separation between oil and free water could be observed on the T2 distribution log from field data.
In the broader concept of Artificial Intelligence, the newly proposed post-processing steps to obtain the oil saturation start by deconvolving the T2 spectra, using blind source separation (BSS) based on independent component analysis (ICA) [Romero, 2016; Romero Rojas et al., 2018]. Based on its T2 peak value —the expected T2Oil peak response— calculated from the prejob planner/simulator, the deconvolution results show that one specific independent component corresponds to the oil, from which the oil saturation was determined.
Results demonstrated the usefulness of NMR logging technology in the characterization and evaluation of this reservoir. Data post-processing based on BBS-ICA enable adequate differentiation between fluid components from T2 spectra. For the reasons above, NMR has been proposed for additional wells in the same field.
The option of using a cased-reservoir analysis system as an alternative to traditional open-hole log analysis was tested as a method to reduce drilling rig time. On a series of five wells in the Gulf of Thailand, an integrated pulsed-neutron system that measures Sigma, inelastic spectroscopy, neutron porosity and density was used to drive the cased-hole analysis. The capabilities and accuracy of the cased-hole results were compared to open-hole porosities and water saturations.
One concern for cased-hole logging is accuracy in this well-completion environment. These wells are completed as monobores with 2 7/8-inch tubing cemented directly into the 6 1/8-inch open hole (no well casing). Initially, the pulsed-neutron porosity coefficients for 5 1/2-inch casing were used with reasonable accuracy. To increase the porosity accuracy, the data from this test served as input to an artificial neural network for determining porosity coefficients in this specific borehole environment.
The project consisted of 12,099 feet of log data including 30 gas sands, 5 oil sands, and numerous water sands in the five wells. The results of this assessment are shown via log examples over specific intervals. In addition, several cross-plots comparing open-hole data to cased-holed data show the results of this evaluation. Graphs of net pay, net effective porosity, net shale volume, and water saturation are shown for the entire project.
Figure 1 is a map of fields in the Gulf of Thailand. Production from the fields operated by Unocal is approximately 1 BCFD of natural gas1 and 33,000 BPD of condensate.2 Since these reservoirs are highly compartmentalized and have limited drainage, it is critical for well-construction costs to be minimized. One technique that is appropriate for this reservoir environment is to drill slim wellbores and complete as monobores with 2 7/8-inch tubing cemented in open hole (no well casing).
To reduce drilling-rig time and lost tool liabilities, a test of the capabilities of a cased-reservoir analysis system was devised. Five wells logged with a standard open-hole "triple combo" were selected to run the Computalog PND™ pulsed-neutron system. In a single logging pass this system measures Sigma, C/O ratio, neutron porosity and density. In this assessment the cased-hole porosities were of prime importance, and the cased-hole gas and oil saturation measurements were of secondary importance.
These reservoirs are predominantly non-marine fluvial sands with interbedded shales and occasional coals. The logged intervals consisted of two different geometries: the upper sections are 8 1/2-inch boreholes with 7-inch casing and 2 7/8-inch tubing (casing-borehole and tubing-casing annuli cemented), the lower sections are 6 1/8-inch boreholes with 2 7/8-inch tubing centralized and cemented. Reservoir fluids are oil, hydrocarbon gas with carbon dioxide, and fresh waters (>5Kppm NaCl). The study was restricted to the shallow part of the productive section by the maximum temperature specification for the PND™ of 300F. At the studied depths (true vertical depth from -3500 to -6190 feet) the reservoir sequences have a low percentage of carbon dioxide (typical >10%).
The low salinity of the formation water means Sigma will have poor oil-to-water resolution. However, the low cross-section of gas can be exploited to make a gas saturation calculation. The Sigma measurement is fairly borehole- independent and standard water saturation evaluations were done using the variable matrix model.3 The Carbon/Oxygen saturation measurement is very borehole-dependent, and characterization for this wellbore environment is discussed in the Results: Saturation from C/O section. Characterization of the porosity measurements is discussed in the following section.