Developed Approach for Better Understanding of Low Resistivity Pay Carbonate Reservoirs

Abstract

Some of the Middle East UAE lower Cretaceous Thamama Carbonate reservoirs are characterized by low resistivity pay phenomena, on which log analysis results in high water saturation computation; on the contrary, production tests confirm dry oil. The Sedmintological investigation on thin sections have shown significant percentage of Micro-porous (occluded porosity or non-efficient porosity) network traps irreducible hyper-saline formation water which responds to a low resistivity, even if the oil is occupying the larger size of porosity (efficient porosity). This paper presents a simple and cost-effective approach for better identification and evaluation of low resistivity pay carbonate reservoirs. The approach utilizes the pulsed neutron capture log (PNCL) in real-time logging in proper identification and formation evaluation in these reservoirs through case studies in the Middle East-UAE Carbonate reservoirs. Through number of wells that have recently been drilled using different types of mud and were logged by LWD PNCL; the results have clearly shown the benefit of minimizing evaluation uncertainties compared to those logged without LWD PNCL. For decades, the combination of conventional logs i.e. neutron, density and resistivity are undoubtedly proven technologies to evaluating homogenous reservoirs. Unlikely, accurate determination of Petrophysical parameters and water saturation evaluation would rather be difficult to show especially in so-called low resistivity pay reservoirs. Since the PNCL (Sigma log) provides a resistivity independent water saturation evaluation, the PNCL has become an integral part of the evaluation program in these low resistivity pay reservoirs, particularly using OBM with relatively high rate of penetration, so that well bore surrounding environment would not be greatly altered (minimum invasion). LWD drill and wipe passes evidently demonstrate that the different between logs can be caused only by reservoir fluid change.

A cohesive approach at estimating water saturation in a low-resistivity pay carbonate reservoir and its validation

“…Additionally, the LRP intervals are commonly identified with high water saturation, which makes such intervals of low interest to the extent that they are discarded as attractive to appraise, particularly when oil prices are low. Typically, LRP zones are characterized by formation interval, with moderate to high porosities, showing extremely low resistivity that are often less than 3 X-m and most frequently encountered in areas with saline formation water (Griffiths et al 2006;Obeidi et al 2010;Worthington 2000;Farouk et al 2014;Uchida et al 2015). While Boyd et al (1995) proposed that the resistivity range is between 0.5 and 5 X-m, several other researchers, like Zhao et al (2000), stated that LRP can be identified by the ratio of the pay zone to the water-bearing zone and this ratio is considered to be in the range of 2.…”

“…While Boyd et al (1995) proposed that the resistivity range is between 0.5 and 5 X-m, several other researchers, like Zhao et al (2000), stated that LRP can be identified by the ratio of the pay zone to the water-bearing zone and this ratio is considered to be in the range of 2. LRP occurs in both clastics and carbonates, while in carbonates, it has been reported to be as a result of either or a combination of deep high saline mud invasion, presence of conductive minerals, presence of microporosity, and anisotropic effect due to drilling high angle wells within thin reservoirs (Griffiths et al 2006;Obeidi et al 2010;Chu and Steckhan 2011). Most especially, tight carbonates are often affected by the deep invasion of conductive mud filtrate, which consecutively affects deep resistivity reading (Souvick 2003).…”

“…One of the techniques proposed to tackle deep invasion problem is running resistivity logging-while-drilling (LWD) since LWD time is less compared to its wireline equivalent (Boyd et al 1995). Obeidi et al (2010) proposed to use pulsed neutron capture (PNC) to evaluate LRP reservoirs. This technique uses chlorine to control the rate of capture of the thermal neutron in the formation.…”

Reservoir Characterization of Carbonate in Low Resistivity Pays Zones in the Buwaib Formation, Persian Gulf

“…LRPZs take place and have reported from both clastic and carbonate reservoirs, in carbonates. It has been reported to be as a result of either or a combination of deep high saline mud invasion, presence of conductive minerals, presence of microporosity, and anisotropic effect due to drilling high angle wells within reservoirs [4] [5]. Typically, LRP zones are characterized by formation interval, with moderate to high porosities, showing extremely low resistivity less than 1 ohm meter.…”

Rock fabric characterization in a low resistivity pay zone from a Lower Cretaceous carbonate reservoir in the Middle East

“…Such micropores can be present within highly micritized skeletal grains (micritic envelopes surrounding fossils and microbial dominated fossils) as well as non-skeletal grains (ooids and peloids). The trapped water within micropores is often highly saline, as high as 200,000 ppm of NaCl (Obiedi et al 2010). The micropores often form an interconnected network containing formation water that provides a continuous path for the measured electric current resulting in a low resistivity reading, and causing a significant underestimation of the true oil saturation.…”