Extra-deep azimuthal resistivity improves navigation in a complex Barents Sea reservoir

Fig. 1. Location of Goliat field, Barents Sea, offshore Norway.

Eni Norge operates Goliat field in the Barents Sea, offshore Norway, Fig. 1. The sand reservoirs are characterized by a large number of faults with relative high dip toward the flank of the structure. The faults are aligned into three main trends.  Geochemical studies and formation pressure data clearly indicate lateral separation into several compartments. In addition, stratigraphic barriers reducing or preventing vertical communication have been identified within Kobbe. This challenging combination calls for horizontal production wells for effective drainage. A major challenge requires the 8½-in. Goliat reservoir section to be initiated in the overlaying Snadd shale. To minimize shale exposure in the landing section, aggressive build-up rates are employed, reducing the length required in shale.

However, a steep approach may lead to deeper penetration in the upper Kobbe sandstone, which can result in unwanted intra-shale drilling. Therefore, the key to successful well placement is early detection of the reservoir top and the accurate mapping of the reservoir architecture, remote to the wellbore.

Previous experience has shown that complex reservoirs can be navigated more successfully using extra-deep azimuthal resistivity tools (EDAR), compared to conventional LWD, to detect stratigraphic boundaries up to 30 m from the wellbore. The development of advanced, multi-component, inversion modeling techniques enhances the interpretation of resistivity data and can accurately provide real-time information regarding reservoir geometry. On Goliat, the EDAR service provided the capability to detect the top of the reservoir approximately 20 m, TVD, and nearly 100 m, MD, before entering the reservoir, enhancing wellbore landing. Extra-deep measurements also reduced the uncertainty in fault detection, because throw could be estimated, based on the displacement of boundaries. The EDAR’s real-time 3D visualization capabilities enabled accurate mapping of formation geometry, maximizing reservoir exposure.

CASE STUDIES 

Fig. 2. Well plan, showing commencement of the build section, with a DLS restriction of 2.5°/30 m landing 1.5 m below the reservoir top. The required distance between the EDAR and drill bit is 7.5 m.

The two case study wells, a gas injector (Well A) and an oil producer (Well B), utilized the EDAR service for
optimal wellbore placement. A maximum dog leg severity of 1.5°/30 m was identified as optimal steering capability in the 8½-in. reservoir sections. Each well has a unique geological and resistivity environment to illustrate the value of the EDAR measurements.

GOLIAT WELL A (GAS INJECTOR)

The gas injector (Well A) example documents a landing scenario and the continuous mapping of the reservoir top and base in a good resistivity contrast environment. After setting surface casing, the 95/8-in. casing shoe was placed in the lower part of the Snadd shale, close to the top of the reservoir, within the seismic uncertainty range. Unstable borehole conditions required a sidetrack from the original well. An RSS equipped with a PDM was used to reduce kick-off time and avoid a trip to lay down the motor. The added length of the motor moved the EDAR measurement point 10 m further back, placing it approximately 30 m above the bit.

Fig. 3. Offset resistivity profile (left) and synthetic EDAR data results generated by inverting expected resistivity values, based on the geo-model and planned wellbore. The cross-section displays the assemblage of all EDAR data inversions along the wellbore trajectory.

The well would follow the structure downward in TVD, then drill horizontally through a fault zone, landing close to the top of the Kobbe reservoir, Fig. 2. Precise positioning of the wellbore in the reservoir represented a challenge, because the depth of the reservoir top was estimated from seismic data with an uncertainty of 10 m, TVD. In addition, the actual dip of the reservoir was uncertain. The combination of these uncertainties and a steep approach could lead to unwanted deeper penetration in the reservoir. If the build could not be initiated early enough, there would be increased risk of drilling into a poorer-quality reservoir, deeper in the stratigraphy.

Conversely, an early build would lead to drilling a longer interval in the overlying shale and could have, in the worst case, resulted in landing the wellbore high in the stratigraphy. The optimal inclination for the landing section was set at 83° inclination, with steering to commence 7.5 m above the reservoir. This would result in landing the wellbore 1 m, TVD, below the reservoir top, Fig. 3.

Fig. 4. Real-time EDAR data, with each bar representing one inversion interval with resistivity color codes (right). The apparent structural dip of the reservoir top is 5°, with first detection of the reservoir by the EDAR tool at 23 m.

To mitigate risks in the landing phase, the EDAR service would be utilized to enable early detection of the reservoir and enable accurate mapping of the formation’s architecture. To achieve the planned build rate, a minimum vertical depth of detection (DOD) distance of 7.5 m of the top reservoir was necessary. To achieve DOD, a specific resistivity contrast needed to be identified between the shale overburden and the top sand reservoir. Shale resistivity was assumed to be 3-ohmmeters (ohmm) and reservoir resistivity to be 100-ohmm. Using this contrast, the DOD was estimated to be 10 m, enough for a safe landing.

During the landing phase, Snadd shale resistivity showed anisotropy with the horizontal component, registering resistivity of 2.5 ohmm (Rh) and vertical resistivity measured at 10 ohmm (Rv). These higher-than-anticipated values, combined with higher-than-expected reservoir resistivity, led to greater actual EDAR DOD than the pre-well model predicted. This resulted in the Kobbe reservoir sand being detected 23 m vertically below the tool, providing approximately 100 m, MD, forward indication for landing the well in the reservoir, Fig. 4. The mapping of the reservoir top over such a length provided information on the apparent formation dip, described as the angle between the formation top and horizontal plane in the wellbore azimuthal direction. For landing purposes, the apparent formation dip was calculated at 5°.

Fig. 5. A three-dimensional view of the earth model. The blue line shows the wellbore’s exit through the roof and the re-entry at the base of the reservoir.

While landing, EDAR data inversions provided a minimum thickness of 8 m for the Kobbe sand. When entering, the Kobbe formation, the base was mapped, and the value was confirmed. When EDAR measurements were taken from inside the reservoir, an intra-shale layer was identified and mapped at the same stratigraphic depth as the presumed base of Kobbe measured during the landing phase. This intra-shale layer likely influenced the inversion results during the landing section. After entering the reservoir, the formation’s top and base were detected and mapped, utilizing EDAR data inversions. The wellbore inclination was kept between 86° and 88°, keeping a consistent distance to the reservoir roof while following the structure, dipping downward at 4°, Fig. 5. While drilling through the fault zones, the reservoir model, offset wells, planned well and real-time data were visualized in a 3D viewer, to ensure all possible input data were accounted for in the decision-making process.

The remaining part of the well was drilled with minor trajectory changes, despite the complex structural setting. The longer-than-normal sensor offset and complex geological environment reduced the feasibility for actively navigating the wellbore. Consequently, EDAR data were used for correlating reservoir with seismic data in real time.

Fig. 6. Pre-well model showing wellbore path descending down through the Kobbe sequence while targeting the lower Kobbe reservoir. The faint black lines above, and below, the well path are the theoretical DOD for the EDAR measurements.

This case study documents the advantages of EDAR, where seismic and extra-deep measurements are used in conjunction to understand the reservoir architecture, and to strategically place the well within the optimal zones of the reservoir. The EDAR data, while drilling in conductive environments, had better accuracy than historically encountered. It shows that evaluating actual resistivity responses, then adjusting as necessary, increased data accuracy.

GOLIAT WELL B (OIL PRODUCER)

Although in the same field, Well B illustrates a different scenario where the reservoir (lower Kobbe), consists of heterogenic channelized sandstones with low lateral continuity. Navigating in this environment would require a high number of trajectory adjustments to reduce the length of shale drilled before reaching the next productive sand. The 8½-in. reservoir section would be approximately 2,000 m, MD, in the horizontal well. The 95/8-in. casing shoe was placed in the overlaying shale, close to the reservoir top, within the uncertainty range of the reservoir model (10 m). The oil-producing interval starts after a mapped fault and extends to TD. 

Fig. 7. Sand formation captured on EDAR data inversions, exhibiting an apparent increase in dip from 3° to 10°.

The reservoir top was confirmed and mapped by EDAR data from the beginning of the section, at approximately 6 m below the EDAR sensor, Fig. 6. Because the objective was to drain the lower Kobbe formation after crossing a mapped fault, the well had to penetrate the upper Kobbe with an inclination of 74° at the top of the reservoir. The EDAR data were utilized to map the reservoir while geometrically landing the well at the targeted TVD depth.

During the planning phase, the landing depth was based on a defined TVD for optimal production, accounting for the gas-oil and oil-water contacts. Landing depth was planned to be achieved just prior to crossing a normal fault, targeting the sand in the foot wall. The intervals prior to, and after, encountering the fault could potentially provide a gain in net sand by utilizing pro-active geosteering.

While approaching landing depth, real-time inversion model from EDAR data identified a formation dip lower than anticipated. Using the planned wellbore trajectory, the EDAR results projected the wellbore to be bed parallel within a shale bed, on reaching 90° inclination. The anticipated fault would change the situation when encountered, but to avoid drilling only shale until crossing the fault, the planned build from 89° to 90° was delayed. While approaching the sand layer below the wellbore, the fault had yet to be crossed. It was decided to decrease inclination to 88°, as a contingency to allow a faster approach to the next sand on the other side of the fault.

Fig. 8. EDAR inversion, used to project formation dip: Case a - fault encountered earlier than expected; Case b - wellbore does not exit the reservoir, due to fault position; Case c - fault is crossed behind the prognosed location, and the wellbore exits through the roof, prior to re-entering at the foot wall.

The anticipated formation dip was estimated to be upward, which was favorable for successful entry into the next sand sequence below the wellbore, Fig. 7. The fault was encountered almost 80 m, MD, after its predicted location. The boundaries mapped by EDAR showed no displacement prior to the location identified as the fault, indicating that the fault crossed at the interpreted depth. EDAR data inversions changed dramatically after crossing the fault. Due to the geological complexity, EDAR interpretation did not detect any sand. It was decided to decrease inclination to 86°, until a sand was identified below the wellbore. A thin-resistive layer (sand) was then detected on the EDAR data inversions. Inclination was then adjusted between 87° and 91°, to minimize shale exposure and maximize reservoir sand in the lateral. After reaching a specific MD, drilling continued at 90° and went through a series of thin sands with an apparent dip of 11°, until crossing the main fault, Fig. 8. After that, the well was kept horizontal with no noticeable major features crossed. This situation agreed with the reservoir model.

CONCLUSION

The use of the VisiTrak service on Goliat demonstrates how the reservoir top and base can be detected simultaneously by EDAR measurements and mapped by inversion. The real-time detection of reservoir boundaries within at least 20 m distance from the wellbore provides unique information which has been used actively for optimal wellbore placement. The information is integrated and visualized, along with the original structural interpretation and geo-model property distribution, to guide correct well placement decisions. The data gathered by EDAR can aid in seismic interpretation and better support the overall reservoir navigation strategy.  

ACKNOWLEDGMENTS

The authors thank Eni E&P Norway and the Goliat field partners for supporting the publication of this article. This article is adapted from SPE paper 174929, presented at the SPE ATCE, held in Houston, Texas, Sept. 28-30, 2015.