Rock-Fluid Characterization Using OBM Contaminated Samples Issues, Challenges, and Solutions

Abstract

Acquisition of representative rock and fluid data from deepwater reservoirs is challenging and costly but is critical for the successful reservoir evaluation. It is common practice to use oil based mud, OBM, for drilling, particularly in deepwater offshore environment. Without proper mud additives and under excessive overbalance, OBM often invades the reservoir and contaminates cores and fluids. Measurements conducted on contaminated samples result in non-representative rock and fluid properties. Therefore, every effort should be made to minimize contamination in the samples. In fluids, mathematical corrections are needed to remove contamination effect and ensure representative fluid properties. In rocks, measurements on samples with minimum contaminations should be validated with those on cleaned samples.High OBM invasion in several cores showed elevated apparent in-situ water saturation as identified by visual examination, CT density mapping, residual brine analysis, and total liquid saturation calculation. Electrical resistivity measurement on "as-received" samples and simultaneous capillary pressure and resistivity measurements on cleaned samples provided correct in-situ water saturation determination.Customized sampling procedures with reliable fluid quality monitoring helped obtain minimally contaminated samples. In addition, novel mathematical techniques allowed correcting fluid samples to calculate representative fluid properties. Field examples are provided to demonstrate the success of these techniques for improved rock and fluid characterization.The results of this study demonstrate the need for reservoir engineers to be closely involved in fluid sampling, coring, and data acquisition stages employing rigorous QA/QC protocols. Customized sampling and coring programs were essential to obtain minimally contaminated rock and fluid samples and rigorous methodologies were critical to correct the data measured on the contaminated samples and determine representative rock and fluid properties. Introduction:Oil-based drilling fluids are in wide use because of their cost and time saving benefits, particularly in offshore environments and in drilling through troublesome shales and salts. The oil based drilling fluids (known as oil based mud, OBM and synthetic base mud, SBM) facilitate trouble-free and safe drilling operation with excellent shale inhibition, penetration rate, and well-bore stability. However, the use of OBM during drilling significantly impacts subsequent well operations such as RFT/WFT testing, coring and fluid sampling and may contaminate the core and fluid samples gathered if adequate steps are not taken to minimize the effects. In addition, obtaining representative rock and fluid data from the contaminated samples requires quantifying the contamination level and identifying a reliable methodology to correct for the effects. In this article, we discuss the critical issues, key challenges, and possible solutions to rock and fluid characterization using the data gathered on contaminated core and...

Contamination of cement slurries with diesel-based drilling fluids in a shale gas well

“…OBMs is a non-aqueous fluid (NAF) system that offers great benefits to drilling through high lubricity and low friction coefficients, superior shale stabilization, excellent fluid loss control, ensure adequate cooling of the drill string, low bit-balling tendency and high associated rate-of-penetration (ROP), etc. (Darley and Grey, 1988;Mike et al, 2000;Berg et al, 2002;Nagarajan et al, 2010). But these benefits of OBMs do not extend to compatibility with cement, unfortunately.…”

Reservoir Fluid Sampling of a Wide Spectrum of Fluid Types Under Different Conditions:

“…The mud invasion in the near well bore regions contaminates the in-situ fluids in this region. A number of techniques are described in the literature for minimizing mud invasion and for adequate cleaning of the near well bore regions (11) to enable capturing least contaminated samples. Even with minimal mud invasion and after extended periods of cleaning, the captured bottom-hole samples may be contaminated to some extent with the OBM filtrate.…”

“…Even with minimal mud invasion and after extended periods of cleaning, the captured bottom-hole samples may be contaminated to some extent with the OBM filtrate. Several mathematical decontamination techniques are available (11) to estimate the clean fluid composition. The industry standard methods of subtraction and skimming methods provide reasonably good estimates.…”

“…In particular, the assumption that the in-situ fluid and the mud filtrate are miscible may not be quite correct with gas condensate fluids. Special methods based on partial miscibility and K-value based techniques (11) may be used in those situations. Complete details of these methods are provided in reference 11.…”