Fracture Aperture Calculations From Wireline and Logging While Drilling Imaging Tools
Carlos MaesoM. PonzianiI. Le NirJosselin KherroubiDaniel QuesadaI. DubourgStefan M. LüthiEvert SlobKelvin FisherLes HoneymanRandy BrownOlfa Zenned
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Abstract 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.Keywords:
Wireline
Logging while drilling
Aperture (computer memory)
Electrical borehole images offer a unique view of the subsurface to geologists and petrophysicists. Images from wireline electrical imaging tools are readily interpreted in terms of key geological characteristics such as structural and stratigraphic features of the formation. Today, advances in logging-while-drilling (LWD) technology allow high-resolution electrical imaging to be successfully applied in water-base. mud drilling environments. Key acquisition advantages for imaging-while-drilling include a better shaped borehole at the time of drilling and 100% circumferential borehole coverage (unlike the pad coverage of currently available wireline resistivity imagers). An important advantage is the opportunity for real-time decision making and related rig-time savings. Images sent to the surface, albeit limited in definition given telemetry restrictions, give an early indication of the angle of entry into a given formation and allow for more accurate/precise geosteering. We present field test results of a new high resolution, electrical borehole imaging-while-drilling tool. We demonstrate its field worthiness and show examples of the quality and accuracy of the images in conductive mud. In a series of controlled runs we have compared the response of the while-drilling tool with its wireline counterpart and with core. We show that the while-drilling images are comparable to the wireline images. In addition, a greater understanding of the geological features is possible because of their full circumferential coverage. Electrical images recorded while drilling show clear occurrences of laminated and disturbed mud rock, cross-bedded and bioturbated sandstone, and composite fractures as well as fracture swarms. This new LWD instrument has been quality-assured through mathematical and experimental modeling. In a laboratory setup, we have simulated the logging of a set of artificial formations with known dip, rugosity, fractures and different mud resistivities. Good agreement is obtained between the mathematical and experimental models. The electrical diameter for the LWD instrument is comparable to that of wireline electrical imaging tools.
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Measurement while drilling
Logging while drilling
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Seismic While Drilling (SWD), specifically encompasses the seismic techniques operated while the drillstring is lowered in the borehole, during effective drilling, during manSuvres or while connecting drill pipes. Two SWD techniques have been used by the industry: - drillbit-SWD, which consists in recording the seismic noise generated by a rock bit under effective drilling on any number of surface seismic sensors. This technique have been used steadily since 1986; - Vertical Seismic Profile While Drilling (VSP-WD), which consists in recording the seismic signal generated by a surface seismic source on seismic sensors integrated inside the downhole Borehole Assembly (BHA). This emerging technique have been operated since year 2000 about, mainly by Schlumberger. Two efficient wireline VSP techniques aimed at gathering geological information potentially useful to the drilling decision making process, and which could rightfully be assimilated to SWD techniques, will not be considered here: - the technique of Tube Logging Conveying (TLC), in particular the TLC-VSP, where a wireline VSP tool is lowered inside the drillstring through a side entry sub at the top of the drillstring; - the standard wireline logging technique of "intermediate VSP", which consists in recording a VSP with a set of wireline logs right before setting an intermediate casing, is used to predict geological features and possible overpressures in depth intervals located hundreds of meters below the intermediate drilled depth, with good success in some geographical regions. The recent developments achieved by IFP and its partners in SWD greatly benefited from the availability of a high rate and real time wireline transmission system while drilling called TRAFOR, allowing for fast field testing of the SWD methods. In the past 18 years (1986-2003), the drillbit-SWD technique practiced by the industry, aimed at a continuous application over the whole drilling depth span, with only a few surface sensors, and without any downhole measurement technology (MWD), reached a very mitigated success rate: although the geophysicists have been intrigued by the large amount of seismic energy imparted to the ground by some types of drillbits, the drillbit-SWD technique fails to yield any substantial results in many circumstances: in the early 1990's, the industry had already gathered an extensive enough experience so as to define the necessary conditions for obtaining any useful drillbit signal: drilling formations sufficiently hard, with a roller cone bit type equipped with milled teeth or inserts, and avoiding the lower range values for the Weight On Bit (WOB) and Round Per Minute (RPM) drilling parameters. The innovative drillbit-SWD technique input from IFP in the past decade is described in the present paper and consists in: - introducing downhole measurements while drilling, in order to understand the downhole process of seismic emission by roller cone rock bits mostly; - improving the mechanical design of the BHA, by integrating a shock absorber element in order to optimize the quality of the seismic signals imparted into the ground and to reduce the amplitudes of undesirable secondary seismic emissions, which considerably complicate the subsequent seismic processing and blur the final seismic image; - designing, manufacturing and validating an operational MWD assisted drillbit-SWD technique through a set of successive, complete and severe field tests achieved through several R&D projects in partnership with the industry; - restricting the application of drillbit-SWD technique to the reverse seismic walkaway configuration to be applied in geological-geographic areas ensuring the best chances of operational and economic success; - integrating the knowledge and know-how of multiple categories of specialists in the various domains of drilling, MWD and seismic, involved in the multidisciplinary applied SWD field operations. In a similar approach successfully applied to the drillbit SWD developments, the technique of VSP-while drilling has been investigated, resulting in several advanced achievements, namely: - definition of downhole seismic sensors able to withstand severe drilling conditions; - field testing the feasibility of the VSP-WD method as a whole, in order to evaluate the quality of the recorded seismic signal in comparison with the equivalent signals from the conventional wireline technique. Depending on the level of seismic signal quality obtained, the domain of application of the VSP-WD technique and the priorities in the successive technological developments to be implemented have been defined; - improving the precision of downhole clock to be embarked in an industrial wireless downhole recorder able to sustain the rough downhole drilling conditions of shock and temperature variations. Last, the present paper overviews the emergence of SWD technique in a brief preliminary history, and is ended with the expression of a few prospective views taking in account the developments achieved by other prominent industrial organizations active in the difficult and promising domains of seismic while drilling technique and field practice.
Wireline
Measurement while drilling
Vertical seismic profile
Logging while drilling
Drill pipe
Scientific drilling
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