Often confusing and unexpected neutron/density (N/D tool responses have been observed from using exotic muds such as petro-free, low-invasion, and formate-based mud to drill wells. For example, it was reported previously that use of petro-free mud in Nigeria, as an alternative to standard oil-base mud, resulted in gas-like effect on N/D in zones with no gas. The effects were present primarily in the oil leg and more often for LWD tools. However, in both cases, often good logs were also obtained and in many instances use of ?appropriate? fluid properties resulted in acceptable interpretation. Recently, using petro-free mud, we noted an anomalous N/D separation in a gas reservoir namely, a higher neutron porosity with the wireline tool, relative to LWD, at a shallow depth and the reverse behavior at a greater depth. In Angola, use of a low-invasion synthetic mud had resulted in inaccurate density readings within the gas leg of two wells with the density porosity showing a 20 pu excursion compared to about a 5 pu decrease exhibited by both a neutron and an NMR device. A survey of data prior to the days of petro-free mud revealed that ?unexpected? N/D separations had also been observed in high GOR reservoirs using standard oil-based mud (OBM. The above issues have raised a challenge in obtaining reliable, unambiguous nuclear tool data using ?exotic? muds and previous work in gaining an insight in order to address the issues have met with limited success. For example, in the case of petrofree muds, initial laboratory measurements of mud properties under room temperature and pressure conditions indicated that the hydrogen index and density of the base fluid are close to those of water and thus could not explain the gas-like N/D response. However, recent lab measurements, under reservoir pressure and temperature conditions, have revealed that petro-free can absorb gas and this raises an array of invasion scenarios that could offer an insight to the log response seen both in liquid-filled reservoirs and gas reservoirs.

Consequently, we have undertaken a systematic assessment of the effect of such muds on N/D response, using modeling and log data, starting with Ester-base mud (EBM, the so-called petro-free mud similar to that used in our operations in Nigeria. Our modeling results to-date reveal that the effects of mud invasion can be very complicated depending on a number of factors, including absorption of gas by the mud, the resulting mud components, temporal evolution of the reservoir fluid composition, especially in a gas reservoir, and possibly on tool design. The insight gained is helping us better understand many of the ?unexpected? or ?anomalous? log results in petro-free, for example how a dynamic fluid property due to evolution of the invasion could have caused the different wireline Vs. LWD responses see at two different depths as noted above. Such insight can possibly help us identify approaches to achieving a reliable porosity interpretation with petro-free mud, and perhaps, ultimately, guide the choice of an appropriate mud in the future. In this paper we review key issues and report on the major findings of our study."

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