Differentiating Gas and Light-Oil in an Invaded Synthetic-Oil-Based Mud Borehole Environment
Authors: Lee Hon Fui , Mukhopadhyay Arunesh , Das Saikat 
Published: March 2015

Abstract
A field case study is used to illustrate the hydrocarbon quantification using 2D NMR to distinguish light oil and gas in the challenging synthetic-oil-based mud (SOBM) filtrate environment. The challenge is due to the low-hydrogen index of the gas and SOBM filtrate invasion, which displaces gas from the volume measured by the NMR tool. In scenarios with light oil, the detection is even more difficult because light oil has properties similar to the SOBM filtrate.
The required measurement sensitivity can be achieved by acquiring the NMR data in two passes: one with parameters sensitive to gas detection and the other with parameters sensitive to liquid-phase light hydrocarbons. Using data from this case study, we show how using the diffusivity versus T2int 2D NMR images significantly improve the NMR sensitivity in detecting small formation oil signals separately from the SOBM mud filtrate and how use of the ratio R=T1/T2app versus T2app 2D method identifies the gas. The technique utilizes the diffusivity and R contrast between different phases. Because the technique involves both the relaxation and diffusion contrasts, the sensitivity and robustness for hydrocarbon quantification is significantly improved compared with the earlier 1D NMR hydrocarbon-detection techniques.
This well was drilled with the expectation of gas; however, initial data suggested the presence of oil which implies a change in completion plans. The acquisition technique provided the information necessary to differentiate the two phases in the SOBM-invaded zone with high confidence and consequence modification of the completion plans.

Introduction
The subject well was drilled in anticipation of gas in a proven field. The field contains light oil, but it is expected deeper than the interval of the present well section. All log data acquisitions were planned accordingly to evaluate the known gas-bearing formation. No formation testing tool was included in the wireline campaign for downhole fluid identification or sample collection. The wireline NMR data was planned to be acquired in the PoroPerm + Gas mode (Chen et. al., 2002), which was designed to identify and evaluate gas in the formation. However, the conventional logs acquired in LWD indicated the probable presence of liquid hydrocarbon. Therefore, the repeat run was redisgned to be acquired in PoroPerm + Light oil mode (Chen et. al., 2002) to identify the presence of any formation oil separately from the OBM filtrate. The data was analysed using various 2D NMR approaches to identify various phases of fluid present in the target reservoir.

Methodology
Modern wireline NMR logging instruments like MReX are capable of acquiring multifrequency, multiple-echo-train data. Each echo train may vary in frequency, interecho time, wait time and acquisition length and the variations are designed according to the underlying fluid types and the logging objectives. A well-designed multiple echo train data set collectively captures both the relaxation time and the diffusivity contrasts for all fluid types to improve the identification and quantification of gas and oil volumes.
Although multiple echo trains can be processed individually, identifying multiple fluids by comparing all the individual T2 distributions is not practical. To take advantage of the multiple-echo-train data sets, it is essential that the different echo trains be processed collectively. This is possible because all echo trains are described by the same governing equation (Chen et. al., 2007):