ABSTRACT: Lithology and Formation Fracture Identification from Measurements of Drillstring Vibrations
HOOD, JAMES, EXLOG, Inc., Houston, TX, ERCILL HUNT, Ercill Hunt and Associates, Houston, TX, and MICHAEL TAYLOR, EXLOG, Inc., Houston, TX
The Hydraulic Fracture Test Site (HFTS) Data Well No. 1 was drilled in the summer of 1991 as part of the Gas Research Institute's (GRI) ongoing research focused on improved design and analysis of hydraulic fractures in tight gas sands.
The well was drilled and evaluated to determine if the Davis Sandstone of North Central Texas would provide a subsurface "laboratory" for comprehensive experimentation in determining hydraulic fracture dimensions. The goal of Data Well No. 1 was to fully characterize the physical properties of the Davis Sandstone through the use of real-time monitoring of drilling and vibration data, whole cores, mini-frac treatments, stress tests, and open hole logs.
Measurement of drillstring vibrations is being used to minimize drillstring damage, optimize ROP and extend bit life. Lithology and formation structural variations, such as fractures, result in varying bit excitation levels and varying drillstring vibration amplitudes and frequencies. To measure drillstring vibrations, an instrumented sub was placed in the drillstring above the kelly.
The instrumented sub was linked by microwave telemetry to a manned data analysis system. Vibration sensors included axial, tangential and lateral accelerometers, and axial and torsional strain gauges. Each sensor was sampled continuously at 2,000 Hertz, analyzed in both time and frequency domains, and merged with data from conventional rig floor sensors.
This paper presents case studies of the relationship between drillstring vibration levels and the drilled formation. For example: During oriented coring after an open hole mini-frac, drillstring axial vibrations showed anomalies which coincided with two thin (30 cm) dirty sandstones in a shale sequence. At a deeper horizon, a 7 meter fractured interval was drilled with anomalously high vibration levels and incomplete mud returns.
Qualitative findings on this well suggest that further work to quantify lithology variation and in-situ stress using drillstring vibrations may be justified.
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)