Looking Through the Borehole, Muddily

New Technology Seeks Better View
In the exploration and development business scientists are after two things — the maximum amount of data to mitigate risk, and the most efficient and economic operations possible.

Unfortunately, those two goals are not always compatible.

Fortunately, new technological advances are getting better at reconciling these problems.

That's certainly true for borehole imaging — particularly microresistivity images, which in recent years have become important, efficient, cost-effective tools providing vital data in subsurface geological investigations.

But there's a drawback of conventional microresistivity tools: They can have problems in oil-base muds, and oil-base and synthetic-base drilling muds have become increasingly popular because they provide increased drilling efficiency and improved borehole stability.

Consequently, microresistivity images must be abandoned in wells drilled with oil-base muds or the mud must be changed, which is expensive and inconvenient — and too often the lack of these images means the loss of critical information.

The search for solutions continues, however, including development of the Oil-Base Micro Imager tool (OBMI). The new tool, offered commercially for the first time this spring after about a year of field testing, is already proving its worth, according to Bingjian Li, a senior geologist with Schlumberger Oil Field Services in Calgary.

Li, along with co-authors. — M. Marshall and P. Goetz from El Paso Oil and Gas Canada, J. Deering and N. Begin from Talisman Energy, P. Cheung and A. Etchecopar from Schlumberger, have written a paper outlining the new tool describing the technology and its geological applications in the structurally complex Canadian Foothills.

"Integrating seismic data with borehole images offers the best solution to structural problems," Li said. "Today we can offer that combined data, even in oil-base mud environments."

Solving Problems

In the past companies tried various methods to provide the same information available from microresistivity borehole images, including full-bore cores, ultrasonic images and/or oil-base mud dipmeters. Each option, Li said, had limitations both in cost and quality.

Image Caption

OBMI and FMI image comparison showing that the westerly dipping low angle dips in green tadpoles picked on OBMI are confirmed by those dips in light blue picked on FMI images. Tracks from left to right present GR in green, OBMI dynamic images with interpreted structural beddings in green sinewave, depth, tadpole plots scaled from 0 to 90 degrees from left to right, and FMI dynamic images.Graphic courtesy of Schlumberger

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In the exploration and development business scientists are after two things — the maximum amount of data to mitigate risk, and the most efficient and economic operations possible.

Unfortunately, those two goals are not always compatible.

Fortunately, new technological advances are getting better at reconciling these problems.

That's certainly true for borehole imaging — particularly microresistivity images, which in recent years have become important, efficient, cost-effective tools providing vital data in subsurface geological investigations.

But there's a drawback of conventional microresistivity tools: They can have problems in oil-base muds, and oil-base and synthetic-base drilling muds have become increasingly popular because they provide increased drilling efficiency and improved borehole stability.

Consequently, microresistivity images must be abandoned in wells drilled with oil-base muds or the mud must be changed, which is expensive and inconvenient — and too often the lack of these images means the loss of critical information.

The search for solutions continues, however, including development of the Oil-Base Micro Imager tool (OBMI). The new tool, offered commercially for the first time this spring after about a year of field testing, is already proving its worth, according to Bingjian Li, a senior geologist with Schlumberger Oil Field Services in Calgary.

Li, along with co-authors. — M. Marshall and P. Goetz from El Paso Oil and Gas Canada, J. Deering and N. Begin from Talisman Energy, P. Cheung and A. Etchecopar from Schlumberger, have written a paper outlining the new tool describing the technology and its geological applications in the structurally complex Canadian Foothills.

"Integrating seismic data with borehole images offers the best solution to structural problems," Li said. "Today we can offer that combined data, even in oil-base mud environments."

Solving Problems

In the past companies tried various methods to provide the same information available from microresistivity borehole images, including full-bore cores, ultrasonic images and/or oil-base mud dipmeters. Each option, Li said, had limitations both in cost and quality.

Previously, the most common source of dip information in synthetic or oil-base mud filled boreholes came from oil-base dipmeter tools. However, a dipmeter with its small number of sensors can only measure dips in simple geological settings. Stratigraphic details, i.e. cross bedding, or fracturs are often difficult to detect.

Li said the OBMI tool can solve these issues.

"It operates in all typical oil-base muds from diesel to synthetic," Li said.

Its four pads provide a total of 20 resistivity measurements with an effective button size of 0.4 inch, giving coverage of 32 percent in an eight-inch borehole, he said. It provides images for formation resistivities ranging from below 0.2, such as those seen in extremely conductive water sands in the Gulf of Mexico, to over 10,000 _m, like the highly resistive carbonates in Canada.

"Calibrated high-resolution resistivities also are provided," he said, "which represent a first in the oil-base mud environment."

According to Schlumberger, the OBMI tool provides details for stratigraphic analysis. Large- to medium-scale sedimentary structures can be potentially identified on OBMI images, which offers useful information for depositional environment characterization.

"The tool's ability to provide the interpreter with the detail required to fully describe small features on or near the borehole wall depends on the size of the object," Li said.

The tool sees fractures and allows their orientation, dip angle and density to be determined — but because the measurement is taken in non-conductive muds, differentiating open fractures from healed fractures, or those filed with resistive minerals, is difficult for interpreters, since both appear as highly resistive.

Various Settings

Li said the OBMI tool has proven valuable in a number of settings, including deep-water logging operations in the Gulf of Mexico and offshore West Africa and structurally complex Canadian Foothills, where the vast majority of wells are drilled using synthetic-base muds.

"One of the major applications of the OBMI tool in the Gulf of Mexico deepwater is to characterize the thinly bedded reservoirs and refine the net pay count." Li said. "An operator ran the OBMI images in a partially cored appraisal well and the OBMI images accurately reproduced the cored interval to fully locate those thin beds. In addition, through using the OBMI data in the uncored interval in the well, the net pay count was increased by more than 50 ft from that determined by conventional log analysis."

The foothills of the Canadian Rocky Mountains — a classic example of a thin-skinned fold-thrust belt — have been another proving ground for the OBMI technology.

Two case studies using the OBMI tool are centered in the northern portion of the Alberta sector of the foothills.

"A regionally significant detachment in the Shunda formation carries Turner Valley dolomite reservoir into thrust related structural traps. The thrusts carrying the Turner Valley continue upwards through the Jurassic and Lower Cretaceous frequently soling out in the Upper Cretaceous Blackstone Formation. The tectonic movement causes intense folding and imbrication in the Jurassic and Cretaceous sediments complicating interpretation." Li said.

Two-D and 3-D seismic data have been used for subsurface structural determination in this area for decades, but defining the deep structures precisely with only seismic data due is challenging due to the highly deformed strata, he said. The clearest picture of the subsurface is obtained when seismic data is reinforced with in-situ dip measurements from the borehole images.

Canadian Study

In one study, Talisman Energy recently drilled a well in the Canadian Foothills to locate a linear northwest-southeast trending structure in the subsurface indicated by surface seismic data.

Three sections were planned for the well — a vertical section with a build at the base, an angled pilot hole and a horizontal leg within the reservoir.

In the vertical section, the operator had to use oil-base mud for borehole stability and decided to use OBMI images for structural information.

In the angled pilot hole, water base mud was to be used as they did always in the previous wells in this area. However, if the OBMI shows surprising dips particularly in the last few hundreds meters of the vertical section, they may have to do some tricky re-designing of the well bore. In the pilot hole to be drilled in WBM, the conventional Formation Microresistivity Imager (FMI) log was to be used.

The horizontal section was to be drilled along the structural strike to maximize penetration of the reservoir and, therefore, production.

"The horizontal leg design was to be based on the integration of structural data from borehole images and seismic data, so the structural information from the lower part of the vertical section OBMI as well as the pilot hole FMI was very important."

The vertical leg of the well was drilled to 3,888 meters and OBMI data analysis for the upper part indicated that the dips are relatively high angle (10 to 40 degrees) and dipping predominantly to the southwest. The structural strike is northwest to southeast, which is close to what was estimated from the seismic data.

An obvious structural domain change was detected at 3,385 meters in the OBMI images; most dips at this interval are low angle (five to 15 degrees), dipping toward the west. Structural analysis shows the strike of the structure below 3,385 meters orients north-south. This was not evident on seismic.

Following the plan, Talisman changed to water-base mud and logged the FMI in the pilot hole to confirm the north-south structural trend. The FMI logged the interval from 3,884 to 4,090 meters, and about four meters of overlap between the OBMI and FMI logs allowed direct comparison of both images in the same interval. The FMI logs confirmed the structural trend of the lower structure observed in the OBMI.

"The regional structural trend was understood from the surface seismic," Li said. "The local variation of the structures around the wellbore has been precisely defined from the OBMI and FMI borehole image data. The two data sets, one with good lateral coverage and the other with excellent vertical resolution, are complementary, allowing Talisman's geologists to acquire a more accurate picture of the structure at the reservoir level."

The second study concerns a well drilled by El Paso Oil & Gas Canada Inc. in conjunction with Suncor Energy, targeting the Turner Valley Formation carbonates in the northern Alberta foothills. There are two major challenges to determine subsurface structures in this area using seismic data alone. First, it is difficult to position the structures precisely due to the anisotropy effect on seismic data. Second, imaging the highly deformed beds can be a difficult task. Therefore, it is important to acquire reliable and accurate in-situ dip measurements from borehole images. To improve borehole stability and drilling speed, El Paso et al decided to drill the vertical hole to the reservoir level using oil base mud and log with OBMI to tie-in their seismic data structurally. If the well was found to be well-positioned, a horizontal extension would be drilled through the reservoir to maximize production. Otherwise, a sidetrack would be needed to reposition the well to a more favorable location and the horizontal well drilled from there. After the OBMI data from the main hole was acquired and analyzed, El Paso understood that the well was not well-positioned structurally. A sidetrack based on the OBMI data as well as re-interpreted 3-D seismic images after correcting anisotropy effect directed successfully the well into the favorable location — near the crest of the structure. In the subsequent horizontal drilling, over 650 m of Turner Valley Formation carbonate reservoir with well-developed natural fracture network was penetrated. The well has been tested with excellent daily production rates.

Acknowledgements

El Paso Oil and Gas Canada Inc., Suncor Energy are acknowledged for the release of the well data and Veritas DGC Inc. Company for allowing the publication of the 3D seismic data. Talisman Energy is acknowledged for the release of the well data. The authors would thank following people within Schlumberger for their support to this project: D. Largeau, G. Mathieu , R. Laronga, P. Montaggioni, O. Faivre, P. Vessereau, M. Garber, M. Lamb, J. Kovacs, A. Kusama, H. Lindsay, L. Silinsky-Kephart.

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