Summary
Formation fluid sampling early in the life of a well ensures that vital information is available for timely input to field planning decisions. For example, in subsea wells, flow assurance is a major concern, and formation fluid samples from openhole logging help operators optimize investment in both upstream and downstream facilities.
When a formation fluid sample is taken from a well drilled with oil-based mud (OBM), sample contamination by the OBM filtrate is a critical factor for the accurate measurement of the sample pressure/volume/temperature (PVT) properties. A technique of monitoring sample contamination from OBM filtrate uses optical means to monitor the buildup of both color- and methane-absorption signals during sampling. The technique provides real-time analysis of sample contamination. Methane detection is essential for condensates and lightly colored crude oils; for such fluids, the color buildup becomes difficult to detect, but the high methane content of these fluids makes possible a reliable methane-based OBM-contamination monitoring algorithm.
Gas/oil ratio (GOR) is an important property of crude oil, and it is a vital input to the design of production facilities. Conventionally, GOR is measured at a PVT laboratory, and it may take many weeks before the laboratory can provide this critical information. In this paper, we describe the development of an in-situ GOR measurement technique, which uses the optical properties of methane and oil components in crude oil. With this technique, GOR can be measured downhole in real time, when the sample is taken, and without requiring phase separation.
Downhole GOR has many advantages over the conventional GOR measurement techniques. It does not require tampering with the sample, which helps the operator maintain the fluid in a single phase during and after sampling. It also can aid in fingerprinting oils from different layers and provides early indications of GOR that can be compared to PVT lab results.
Both the OBM contamination monitoring and the GOR algorithms work well for most crude oils. However, for heavy (dark) oils, the contamination prediction from the methane component and the GOR prediction become unreliable because of the color effect. In this paper, we describe the methodology for downhole GOR measurement, and we provide details of a decolorization technique to remove the color effect of dark oils from the methane, oil, and base channels in a downhole optical fluid analyzer tool. This technique significantly improves real-time contamination monitoring and GOR prediction results for dark oils.
Introduction
Real-time estimation of sample contamination by drilling-mud filtrate is critical for the collection of representative hydrocarbon-fluid samples in wells drilled with OBM. The hydrocarbon sample may become useless if the contamination is too high (typically above 10 to 15% for crude oils or 1 to 3% for gas condensates). In-situ sample OBM contamination can be predicted in real time by a downhole optical fluid analyzer tool, which is used as a module of a formation testing tool (Mullins and Schroer 2000; Smits et al. 1995; and Crombie et al. 1998). This is accomplished by using a technique of monitoring OBM contamination, which is based on measuring the change of methane content and color in the flowline as cleanup with the downhole pump proceeds and progressively larger fractions of formation fluid replace the OBM filtrate.
An accurate value of the GOR is important for many applications, including crude-oil typing and production facilities design. Conventionally, GOR is measured in a PVT laboratory by flashing the crude oil and then measuring the volumes of the gaseous and liquid phases at standard conditions (1 atm and 60°F). It may take many weeks before the laboratory can provide this critical information. The downhole optical fluid analyzer tool has a methane channel and an oil channel, which cover the methane absorption peak and oil absorption peak, respectively. We have developed an in-situ GOR measurement technique that derives GOR from the optical density (OD) ratio of the methane channel and the oil channel. Thus, GOR can be measured downhole in real time, when the sample is taken, and while keeping the sample intact. Downhole GOR is valuable in providing an early confirmation check for subsequent laboratory PVT analysis. The downhole GOR measurement also aids fingerprinting oils from different layers and helps the operator maintain the fluid in a single phase during sampling.
Both the OBM contamination monitoring and the downhole GOR techniques work well for the majority of light- to medium-colored crude oils. However, when these two techniques are applied to heavy oils, the color absorption of the crude extends to the near-infrared region (NIR) and covers the methane and oil molecular-vibration peaks. If not corrected for, this would result in errors in the methane-based contamination prediction and GOR prediction.
This paper describes a decolorization algorithm to remove the color effect from the methane and the oil channels. This algorithm is based on the exponential decay of color absorption toward the longer wavelengths in the NIR region. After decolorization, the methane and oil channels contain only the molecular-vibration absorptions of methane and oil, which are then used to derive an accurate crude-oil contamination value and GOR.
The examples described here involved OBM. It should be noted that all the techniques for GOR calculation mentioned in this paper can be, and have been, applied successfully to sampling in wells drilled with water-based mud.
Vaporization order and burning efficiency of crude oils during in-situ burning on water
