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Six horizontal wells were drilled into the Tertiary Chatt Sand reservoir of the Breitbrunn gas field in Bavaria, Germany. The purpose of this campaign was to develop part of the depleted reservoir into a gas storage sand. A detailed geological and petrophysical study was prepared prior to drilling and resulted in the identification of high quality reservoir layers that were targeted by the horizontal wells. Despite the simple anticline structure of the field, geometric drilling was ruled out due to remaining geological and directional uncertainties. The geosteering approach adopted relies on real-time GeoVision Resistivity (GVR*) resistivity images, which were used for the first time ever during this drilling campaign. The image data are compressed downhole and transmitted to the acquisition computer on the rig where they are decompressed and analyzed. The images allow the precise placement of the borehole relative to the geology, which "keeps the bit in the sand". Layer heterogeneity like tight streaks, concretions or patchy porosity can be identified as such, and are not interpreted as a different layer entered by the hole, which would lead to a wrong geosteering decision. Logging-While-Drilling (LWD) azimuthal data are acquired during drilling and also during wash-down passes which follow a bit change. A comparison of these time-lapse data sets can provide invasion profiles through time and around the borehole.
The Breitbrunn/Eggstatt gas field was discovered in 1975 in southern Bavaria, Germany (Fig.1). The NE-SW striking anticlinal structure covers about 30 square kilometers and consists, from top to bottom, of sands A through H with sands A through D being the original gas producers. The lower sands are wet. The individual reservoir layers range in thickness from about 5 to 15 meters and are separated by impermeable calcareous shales. The immature sands were deposited immediately north of the rising Alpine orogen during the Tertiary. Mineralogically they consist of carbonate sand, principally dolomite, quartz and micas. The sands were deposited in a fluvial/deltaic setting.
Initial production was by vertical wells drilled on the top of the structure. After depletion of the reservoir, layers A and B were converted into a gas storage reservoir with layer B being the storage sand while layer A functions as monitoring unit for gas leak detection. The demand of natural gas during winter months in the region led to the drilling campaign discussed here with the aim of increasing the storage capacity of the reservoir. The remaining original gas sands C and D were targeted for storage development as it was known that they possess sufficient porosity and permeability although with greater geological and petrophysical heterogeneity than the upper two sands. Reservoir quality in this field deteriorates from the top sand downward.
The drilling phase of sands C and D was preceded by geological, petrophysical and geomechanical studies.
The goal of the geological study was to achieve a structural accuracy of 0.1%, which translates into a depth inaccuracy of maximal 1.5 meters. This was achieved by re-surveying well locations and using directional surveys from casing runs for adjusting all log-derived marker picks to a common baseline. The vertical pilot well and the subsequent horizontal development wells confirmed that this depth accuracy was achieved.
Halliburton introduced 3D reservoir mapping, a new logging-while-drilling (LWD) capability that provides a detailed representation of subsurface structures to improve well placement in complex reservoirs. In geosteering applications, the technology maximizes contact with oil and gas zones while mapping the surrounding formation to identify bypassed oil, avoid drilling hazards, and plan for future development. "This unique technology moves beyond layered reservoir models to full 3D characterization of the reservoir, enabling accurate well placement," said Lamar Duhon, vice president of Sperry Drilling. "In complex formations, visualizing data in a 3D environment helps operators significantly enhance reservoir understanding to drive better drilling decisions and maximize asset value." The 3D capability originates from downhole measurements taken by the EarthStar ultradeep resistivity service, an LWD sensor that identifies reservoir and fluid boundaries up to 225 ft from the wellbore.
Maeso, Carlos Jeronimo (Schlumberger) | Ponziani, Michel (Delft University of Technology) | Le Nir, Isabelle (Schlumberger) | Kherroubi, Josselin (Schlumberger) | Quesada, Daniel (Schlumberger) | Dubourg, Isabelle (Schlumberger) | Luthi, Stefan M. (Delft University of Technology) | Slob, Evert C. (Delft University of Technology) | Fisher, Kelvin (Endeavor Energy Resources) | Honeyman, Les (Endeavor Energy Resources) | Brown, Randy (Endeavor Energy Resources) | Zenned, Olfa (Schlumberger)
The presence of fractures in reservoirs can have a large impact on short and long term production. Electrical imaging tools have a long history in the identification and quantification of fractures in boreholes drilled with water base muds. These tools are particularly sensitive to conductive fractures. The width (also known as aperture) of open fractures is calculated by a well-established equation, relating the fracture width to the excess current measured by the imaging tool (Luthi and Souhaité, 1990). Both mud resistivity and background resistivity of the formation need to be known or measured. The equation was derived from 3-D finite element modeling of the borehole imaging tools of the time.
Recent work has revisited the fracture aperture calculations. The work has verified the approach for electrical imaging from modern wireline tools and extended the principle to Logging While Drilling (LWD) tools. A twofold approach has been taken for the work. Firstly 3-D finite element modeling had been carried out. This includes detailed modeling of the tool sensors’ geometry and the analysis of the electromagnetic responses when the sensors are passed in front of a range of fracture widths. The modeling is complemented by a series of physical experiments carried out at Delft University. Setups utilized either a wireline pad or an LWD sensor from the relevant imaging tools. The sensors were traversed across two blocks separated by a precisely measured gap. Measured excess current relates to the fracture apertures and verifies the theoretical modeling work. This combined work confirms the equation for the fracture aperture calculation. In addition the coefficients for both the modern wireline and LWD electrical imaging tools are determined.
Workflows for the quantification of conductive fractures identified on borehole images have been refined and implemented. Fractures are commonly not continuous across the borehole. The workflow includes a fast automatic extraction of both discontinuous and continuous fracture segments. Fractures are grouped into sets based on relevant criteria (such as orientation). Apertures are calculated using the relevant tool coefficients. The fracture density and porosity are then accurately computed along the well. This enables quantification and characterization of the fracture network, including a fast and easy recognition of intervals with specific aperture or porosity ranges. The workflow is demonstrated by examples.
Use of magnetic-resonance-image (MRI) logging is growing as a logging while drilling (LWD) tool. The use of chemical nuclear sources downhole has been a logistical and management headache. MRI, by measuring in real time the free-fluid, capillary-bound-water, and clay-based-water volumes, offers an alternative, lithology-independent porosity measurement in complex lithologies. It can be used for geosteering and geostopping when sufficient productive formation has been exposed to the wellbore. Like most measurements, at an initial phase there are specialist applications that are more susceptible to realizing the value of magnetic-resonance logging.
Logging while drilling (LWD) refers to the addition of wireline-quality formation measurements to the directional data of a Measurement While Drilling (MWD) service. Although attempts to deliver LWD serices date back to the 1920's, the first viable tools were by J.J. Arps in the 1960's, but these did not become a commercial service. The growth of MWD in the late 1970's and early 1980's delivered the first commercial LWD services by the major service providers. The initial tools were natural gamma and resistivity, and these made geosteering possible, as horizontal drilling grew. Information is returned to the surface using the same methods as MWD telemetry options.