Seismic While Drilling (Swd) Techniques with Downhole Measurements, Introduced by Ifp and Its Partners in 1990-2000

Abstract

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 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 RD - 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.