Mapping CO2 in Real Time With Downhole Fluid Analysis in the East Irish Sea

This paper describes the first successful attempt on the continental shelf offshore UK to map carbon dioxide (CO2) in real time while logging during a drilling campaign in the East Irish Sea.

This paper describes the first successful attempt on the continental shelf offshore UK to map carbon dioxide (CO2) in real time while logging during a drilling campaign in the East Irish Sea. Reservoirs in this sea’s basin contain varying proportions of CO2, nitrogen (N2), and hydrogen sulfide (H2S), in addition to oil and methane. Two of these wells develop the Rhyl gas field. Downhole-fluid-analysis (DFA) technologies were deployed with a wireline-formation-testing (WFT) tool to measure CO2 content accurately downhole.

Introduction

The Rhyl field was discovered in 2009 and received development approval in 2012. It is located 11 km north of the North Morecambe field. The North and South Morecambe fields were discovered in the 1970s, with some 7 Tcf of gas initially in place. Production from the Rhyl field extends the longevity of these assets.

Vertical and horizontal variations in CO2 content in the Rhyl field were assessed across the Triassic Ormskirk sandstone, the upper member of the Sherwood sandstone group. The Ormskirk sandstone formation represents the principal reservoir target in the East Irish Sea, comprising high-porosity aeolian and fluvial sandstones with variable grain size and playa mudstones.

Gas Composition in the Rhyl Field

The composition of the gas found in Rhyl includes both hydrocarbon and nonhydrocarbon components such as N2 and CO2. The N2 is derived from late-stage hydrocarbon generation, while isotope data indicate that the CO2 has a magmatic origin. It is believed to have been exsolved from the magma of a series of Tertiary dolerite intrusions into the Ormskirk sandstones in close proximity to the Rhyl field. The samples collected during the drillstem test conducted on exploration Well 113/27b-6 provided several qualitative indications of a higher CO2 content than was originally expected from the correlation with the nearby North Morecambe field: a high gas density measured at the separator and difficulty in sustaining a flare.

Developing High-CO

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Reservoirs

On average, the global risk of encountering concentrations of CO2 greater than 1% in a gas accumulation is less than 1 in 10, and the risk of encountering concentrations of CO2 higher than 20% is less than 1 in 100. A timely identification of CO2 in a hydrocarbon reservoir is, hence, essential for several reasons. First, when CO2 is abundant, it is often so abundant that it can kill the prospect economics. Second, CO2 occurrence in hydrocarbon-bearing formations presents a challenge to the valuation and subsequent prospect development of the hydrocarbon accumulation. Corrosion is a major concern affecting capital and operational expenditures, but CO2 also denotes an issue for health, safety, and the environment. In this respect, timely identification and quantification of CO2 are needed to enable reserves and sales-gas (and hence revenue-stream) estimates to be made, schedule and optimize gas processing, and plan for any eventual CO2-emission taxes and CO2-storage or -sequestration costs.

The specific challenge of CO2-mapping in the Rhyl field is related to the structural complexity of the field itself: The presence of faults, low-permeability sandstone beds, and laterally extensive playa-mudstone beds could potentially make the vertical and horizontal distribution and equilibration of CO2 in the Ormskirk sandstones difficult to predict. Additionally, the highly reactive nature of CO2 may result in significant concentration changes before reaching an analysis facility. Optimizing the fluid-sample-acquisition program to account for existing fluid complexities is impossible without real-time analysis.

Initial Evaluation

The presence of CO2 was identified during the post-well analysis of the 113/27b‑6 exploration well in 2009. No downhole gas sampling or fluid analysis was planned because the gas composition was expected to be similar to that of the North Morecambe field. During testing, difficulty in lighting the flare and the high gas density measured at the separator both indicated the presence of CO2. High CO2 content was confirmed subsequently during pressure/volume/temperature (PVT) analysis of the surface samples, and isotopic analysis confirmed a magmatic origin.

Initial petrophysical analysis of the reservoir interval identified the presence of an upper and lower Ormskirk sandstone unit, with porosities up to 24% and permeabilities up to 2 darcies and separated by a laterally continuous mudstone unit (Centurion mudstone) offset by faults. Downhole formation pressures were acquired across the Ormskirk sandstone, suggesting vertical pressure communication across the mudstone.

DFA and Real-Time CO2 Mapping in Rhyl Field Development Wells. Because of the elevated risk involved with high-CO2 reservoirs, a key objective of the development plan was to acquire further data on potential variations in CO2 concentration and on vertical and horizontal distribution of CO2 throughout the field. Traditionally, the preferred method to evaluate the presence of CO2 within downhole samples has been through standard PVT analysis. However, because of the nature of this project and its development through an appraisal strategy, information on CO2 concentration and any potential vertical variation needed to be available in real time, while the WFT tool was still in the hole. This would ensure that a first evaluation of the hydraulic connectivity across the Ormskirk sandstone and Centurion mudstone was possible as soon as the data were acquired, thus maximizing the value of the data acquired by enabling real-time evaluation.

The latest generation of DFA sensors run with the WFT tool was selected as the most appropriate technology to achieve the proposed objectives. This technology combines a series of sensors and allows the application of optical principles for continuous analysis of the fluids passing through the WFT-tool flowline. Two optical spectrometers, combined with fluorescence and composition--independent density sensors, allowed the measurement of gas composition and density with a high level of confidence, thereby assessing any vertical and horizontal fluid variation in real time (Fig. 1). These data were then integrated with pressure measurements obtained on the same wells, and vertical and lateral connectivity and CO2 variations were then assessed.

jpt-2014-07-mappingco2fig1.jpg
Fig. 1—Real-time fluid composition and density measured by the DFA dual spectrometers deployed with the WFT tool. Note that the Rhyl is predominantly composed of CO2 and methane, with traces of C6+, confirmed by the laboratory analysis (Track 1). Real-time fluid density is displayed on Track 2. The bottom track (Track 3) confirms the excellent quality of the signal analyzed by the spectrometers.

With the previous generation of DFA tools, CO2 has been a difficult component to quantify. This is because water has a broad and strong absorption peak that lies in the same region of the infrared spectrum as CO2. The presence of small amounts of either formation water or water-based-mud filtrate can potentially swamp the CO2 signal. The dual-spectrometer system of this latest generation of DFA tools, however, covers wavelengths from 400 to 2100 nm and uses a number of channels in the near-infrared (NIR) range tuned to the response of CO2 and to the water vibrational mode, in order to compensate for the effect of water. The quantification of CO2 in real time is therefore extremely accurate.

Fluid density was measured by a different sensor independently from fluid composition. The ability to measure fluid density independently from composition became key when calculating the concentration of N2 in the pumped gas. In fact, N2 has no absorption within the range of wavelength of the DFA dual-spectrometer tool; therefore, its presence cannot be detected or quantified by visible or NIR spectroscopy. A different approach has been used in this case, and the concentration of N2 has been calculated indirectly.

Estimating N

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Concentration

An indirect approach for determining N2 concentration has been used in this project. Initially, the concentration of hydrocarbon and CO2 was measured by the DFA dual-spectrometer sensor deployed with the WFT tool. The corresponding density for gas of that composition (typically C1, C2, and CO2) was then calculated by solving an equation of state (EOS) at specific pressure and temperature conditions. The measured fluid density was compared with a density calculated by the fluid composition measured by the DFA sensor. The difference between the two densities (the measured and the calculated) was attributed to the presence of nonhydrocarbon components (i.e., N2). A new fluid composition was then assumed. Density calculation with the EOS was then repeated for this newly assumed fluid, to check the consistency. This process was iterated until the assumed fluid and the EOS came to good agreement. This method of solving the EOS iteratively allowed the building of a reservoir/field-based ternary diagram that could eventually be used to estimate the concentration of N2 without solving for the EOS at each sampling station.

Preserving the Original Concentration of CO

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in the Downhole Samples

CO2 is a very reactive gas by nature and has the tendency to react with the metals present on the WFT tool (flowlines, sampling bottles, and pumpout units). In order to reduce the scavenging effect of this gas and preserve a representative concentration until the sampling cylinders are delivered to the laboratory for full PVT analysis, a few technical solutions have been adopted.

The WFT-tool string has been configured in reverse-low-shock sampling, a setup ensuring that the length of the path between the probe and the sampling bottles is minimized. This reduces the contact between the CO2 and the various WFT modules and minimizes any scavenging effect.

The chances of failures in the pumpout unit because of potential embrittlement caused by the CO2 were minimized by use of CO2-resistant displacement units. The sampling cylinders have been coated with a CO2-resistant material that ensured that no scavenging effect took place inside the bottles.

Results

More than 170 pressure measurements have been taken within the Ormskirk sandstone formation on four different wells. Analyses of formation pressure and gradients were integrated with DFA performed at 13 different depths. A total of 20 downhole fluid samples were collected from the four wells, allowing an accurate description of the gas and aquifer-water composition.

The use of the latest DFA technologies allowed optimization of the number and location of collected samples. In 30% of the cases, the information coming from the dual spectrometer was sufficient to fully characterize the pumped fluid; thus, no further sampling was required.

DFA did not show any substantial vertical or lateral variation in gas composition, and the analysis of the pressure data and the related gradients confirmed the vertical and horizontal connectivity of the reservoir. Pressure and fluid data suggest that the presence of the Centurion mudstone in the upper part of the Ormskirk formation does not represent a vertical barrier to gas flow across the reservoir. There are two likely reasons for this. First, the extremely low viscosity of the gas, and thus its high relative permeability, allows the gas to move easily across the reservoir even in the presence of low- or relatively-low-permeability barriers. Second, the Centurion mudstone is known to be fractured to some extent. These fractures represent highly permeable streaks across which the gas could move, allowing the CO2 to equilibrate.

The gas found in the Rhyl field is mainly composed of C1–C6 and CO2, with some minor concentrations of N2. The variations between the various wells measured by the DFA sensors are minimal and most likely attributable to measurement accuracy or very localized subtle variations rather than to real compositional differences. No H2S has been found in this field.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166497, “Mapping CO2 in Real Time in Hydrocarbon Reservoirs With Downhole Fluid Analysis: First Successful Experience in the East Irish Sea, UK Continental Shelf,” by B. Quayle, S. James, and M. Quine, Centrica Energy, and I. De Santo, P. Jeffreys, and J.Y. Zuo, Schlumberger, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September–2 October. The paper has not been peer reviewed.