A novel multicomponent induction resistivity tool increased oil-in-place estimates in low-resistivity pay intervals in Oman.

Correct evaluation of low-contrast pay scenarios is a major concern in today's frontier exploration markets such as the Gulf of Mexico, West Africa, the Asia-Pacific region, the North Sea and the Middle East. E&P managers have at their disposal a new multicomponent induction resistivity tool to complement open-hole logging. This new generation of resistivity measurements solves a subsurface problem that has plagued geoscientists for decades. This technology yields more accurate and reliable reservoir description, helping to maximize reservoir production from low-resistivity, low-contrast, shaly sand pay zones. Impressive increases in estimated oil initially in place (OIIP) were recently realized in the Marmul Haima West reservoir, providing a look at how oil companies can economically benefit from this technology.
Thinly laminated sand-shale sequences typically encountered in deepwater turbidities often are overlooked or mistaken as water-bearing sands. More accurate and reliable petrophysical quantification is crucial for the economic decision to explore, develop and produce these reservoirs. Traditional induction logging has experienced a technological breakthrough with the introduction of the 3D Explorer™ Induction Logging Service (3DEX™), a multicomponent induction tool complementing resistivity measurements. Shell Technology EP and the Houston Technology Center of Baker Atlas and Hughes Inteq have jointly developed and field-tested this technology.
Measurements provided by the 3DEX instrument have the potential to impact reservoir economics and OIIP significantly through detection and evaluation of low-resistivity pay zones such as those in the Marmul Haima West reservoir, in the southeastern region of the South Oman salt basin within the concession area of Petroleum Development Oman (PDO). The multicomponent induction tool measurements provide vertical and horizontal resistivity information that allows a more accurate and reliable petrophysical evaluation of low-resistivity, low-contrast pay zones.
In the Marmul field, the characterization of the reservoir formation based on traditional resistivity measurements led to inaccurate, unreasonably low estimates of OIIP. The 3DEX instrument has been used to log three wells in this reservoir. The multicomponent induction instrument measurements quickly highlighted laminated sand-shale sequences with potential hydrocarbon productivity. Petrophysical evaluation using the new vertical resistivity measurement indicated a substantial increase in OIIP. This supported earlier results from evaluation work carried out using traditional core analysis, well testing, carbon-oxygen logs and capillary pressure measurements, along with standard wireline logs. Field redevelopment is taking place with improvements in the reservoir production performance and a more accurate reservoir description. The overall re-evaluation has resulted in a 225% increase of field OIIP to 1.27 billion bbl of 23° API crude. Full-field waterflooding is forecasted to increase the ultimate recovery factor from 6% to 24% of OIIP, amounting to 233 million bbl of incremental reserves.
Evolution of induction logs
The introduction of the multicomponent induction resistivity tool opens a new chapter in the history of induction resistivity measurements, which spans several decades. The technology evolved from simple first-generation transmitter-receiver arrangements to hardware-focused induction phase and amplitude measurements. Improvements in vertical resolution and depth of investigation were achieved by choosing interconnected arrays of transmitter and receiver induction coils.
During the 1980s, the next generation of induction tools was introduced to the industry. An array of transmitter-receiver induction coils performed measurements at multiple frequencies and spacings, providing enhanced vertical resolution and a more detailed resistivity profile. With sensors aligned along the borehole axis, these array induction measurements are limited to a single dimension. These measurements are satisfactory for the evaluation of sand-producing zones that are at least several feet thick. However, for an accurate evaluation of low-resistivity pay in thinly bedded or laminated reservoirs, the addition of a vertical resistivity measurement provides much better sensitivity to the presence of hydrocarbons.
The latest advance in induction logging technology comes with the introduction of 3DEX. This instrument performs resistivity measurements in three dimensions, and the data interpretation yields horizontal and vertical resistivities that lead to reliable identification and accurate petrophysical evaluation of low-resistivity pay. It provides complementary data and additional information when used in combination with standard induction logs.
Principle of operation
Figure 1 shows a vertical well in a laminated sand-shale sequence. The conventional induction coil array is coaxial with the tool and borehole, inducing a current in the plane parallel to the lamination sequence to measure the horizontal resistivity. This horizontal resistivity is heavily biased toward the low-resistivity shale and is insensitive to the hydrocarbon-bearing sand resistivity. As an example, in a 75% net-to-gross case with shale and sand resistivities of 1 and 10 ohm-m, respectively, the horizontal resistivity is suppressed to 3 ohm-m, while the vertical measurement still reads about 8 ohm-m. When petrophysical analysis is done using conventional horizontal resistivity models, potentially productive sand-shale sequences cannot be reliably quantified.
In many cases the use of an improper petrophysical evaluation model based on conventional horizontal resistivity may result in an underestimation of reservoir producibility and hydrocarbon reserves by 40% or more. To overcome this problem, the multicomponent induction tool makes additional measurements. Horizontally mounted coil arrays are aligned with their axes perpendicular to the tool and borehole axis to measure vertical resistivity. These arrays induce current that flows across the laminated sand-shale sequences, which are far more sensitive to the hydrocarbon-bearing sand resistivity. A vertical and horizontal resistivity value is obtained by incorporating these direct measurements from the coil arrays.
Production challenges
More than 300 wells have been drilled in the Marmul field. Hydrocarbon production is from the Mahwis, Al Khlata and Gharif formations at depths ranging between 1,650ft (500m) and 2,625ft (800m).
When comparing saturation values calculated from core capillary pressure data with those from conventional wireline resistivity logs and actual production tests, the open-hole wireline data were found to be an unreliable indicator of hydrocarbon saturations. Resistivity from conventional induction logs and calculated saturations for the oil leg show large variations and no correlation to porosity or shaliness. The saturations derived from capillary pressure curves are much higher and more consistent. Dry oil is produced at high rates from intervals with high and low resistivity.
The presence of thin, mica- and pyrite-rich, conductive shale layers within the reservoir sands resulted in a suppression of conventional resistivity log responses and, consequently, a lower estimate of hydrocarbon saturation over these intervals. A conductivity increase of 0.1 siemens/m due to these conductive layers combined with a relatively fresh formation water (about 1 ohm-m) can result in apparent hydrocarbon saturations dropping from about 80% to very small values. During field appraisal, the lower resistivity intervals were considered nonpay, and only high-resistivity sands were perforated. The early resistivity-log-based field appraisal results were contradicted by analyses of core samples from those resistivity-suppressed intervals, clearly indicating oil staining.
Further confirmation of resistivity suppression came from directional resistivity (vertical and horizontal) measurements performed on a water-saturated cubic sample under realistic triaxial stress. These test results indicated the presence of resistivity anisotropy; the horizontal conductivity was 0.12 siemens/m higher than vertical conductivity. This anisotropy effect is illustrated with differences between vertical resistivity (Rv) and horizontal resistivity (Rh) in Figure 2.
In-situ measurements in three newly drilled Haima West wells logged with the 3DEX instrument confirmed this resistivity anisotropy. Saturations derived from 3DEX data (Sov) were far more consistent with upper limit (SoCCup) and lower limit (SoCCdn) capillary-pressure-derived saturations than conventional resistivity-log-derived saturations (Soh).
3DEX measurements clearly highlighted low-resistivity, laminated sand-shale sequences with potential hydrocarbon productivity. This resulted in a significant OIIP increase and led to an economically successful field redevelopment. This new technology may have enormous economic impact when used in exploration wells during reservoir appraisals and field redevelopments.
Acknowledgements
The authors thank PDO and the Oman Ministry of Oil and Gas for permission to publish this information.