Borehole Seismic Processing Summary Checkshot Vertical Seismic Profile

Transcription

1 Borehole Seismic Processing Summary Checkshot Vertical Seismic Profile COMPANY: Gaz de France WELL: G 14-5 RIG: Noble G.S. FIELD: G 14 LOGGING DATE: COUNTRY: Ref. no: 10-MAR-2005 The Netherlands, Off shore EW Data processed: E. Wielemaker, A. Voskamp Report prepared by: E. Wielemaker, The Hague Report approved by: T. Ter Burg Date: March 30, 2005

2 Legal notice This report was prepared by Schlumberger, Data and Consulting Services Division (DCS, The Hague, The Netherlands) at the request of Gaz de France. The report is Schlumberger DCS s interpretation of the information provided to Gaz de France and the recommendations and conclusions contained in the report are based on inferences from measurements and empirical relationships and assumptions, in respect of which there may be a number of possible interpretations and conclusions. All representations and warranties, express or implied, in respect of this report are hereby excluded. Schlumberger DCS accepts no liability for any use, which Gaz de France may make of the information, recommendations or conclusions contained in this report or for any decisions which Gaz de France may take as a result of this report. 2

3 Contents 1 INTRODUCTION DATA ACQUISITION RAW GEOPHONE AND SURFACE SENSOR... 5 SURFACE SENSOR RECORDS... 5 DOWNHOLE SENSOR RECORDS TIME PICKING ON THE HYDROPHONE AND TRACE SELECTION STACKING, TIMEPICKING AND DEVIATION CORRECTIONS REMARKS AND CONCLUSIONS RESULTS CD-ROM... 6 Processing Flow Chart - 7 APPENDIX A - Deviation Data/Time Depth Relation and Header Information APPENDIX B - QC plots & Checkshot Display 3

4 1 Introduction A borehole seismic survey was carried out on March 10, 2005 on the well G14-5, with the objective of acquiring a reliable time-depth relationship that can be used for subsurface modeling and time-to-depth conversions. The well profile is near vertical with a maximum deviation angle of 1.67 degree. The seismic source was located 81.4 m from the well head, the geometrical effect of raypath obliquity needs to be taken into account to produce vertical travel times. 2 Data Acquisition The well G14-5 is located in the G14 field, North Sea, Dutch Sector. The borehole seismic survey consists of a Checkshot recorded in open and cased hole sections. The downhole tool is the CSAT (also referred to as CSI - Combinable Seismic Imager Tool) that is provided with 3-component sensors of accelerometer type (GAC Geophone Accelerometers) and a Gamma Ray cartridge. The tool was set at the following depth stations: 1500 m, 2000 m, 2000 m and 2600 m while logging down 2925 m to 2814 m in the open hole interval (6 levels) 2754 m to 1599 m in the single casing section (10 levels) 1553 m to 1100 m in multiple casing (4 levels) At every depth station, a minimum of 3 shots have been fired, to increase the signal to noise amplitude ratio, by summing up the original traces. Shots were fired using a tuned airgun array located at the surface and consisting of three Sodera G-Guns of 150 in 3 each, operating at the pressure of 120 bars. The gun array is located in a single position at a short distance from the well head (81.4 m with azimuth of 279 deg). The guns are immersed at a depth of 4 m depth below the sea surface. A hydrophone (MP-24C), used as timing reference, is positioned below the center of the gun array at a depth of 8 m below sea level. The firing of the source was controlled by the Schlumberger MCM recording system on the rig; from where the data quality was constantly monitored in real time. The acquired data were recorded to 4 mm DAT tape in DLIS format and was provided to the Schlumberger office in The Hague. 4

5 3 Raw geophone and surface sensor Although the well site acquisition system adequately prepares the recorded data for a Quick Look seismic product, a full re-evaluation of the raw shot data is necessary. This is called Well Seismic Edit (WSE) and is described in sections 3, 4 and 5. Surface sensor records The hydrophone plots (figure 1) contain the hydrophone data used to establish the absolute shot time. All shots are sorted according to cable length (measured depth) and CABLE LENGTH is displayed to make a correlation with the downhole geophone depth levels. The ACQUISITION SHOT NUMBER is also indicated. Each trace is scaled relative to its maximum amplitude for plotting. Downhole sensor records The downhole receiver plots (fig. 2) comprise the downhole geophone records of the tool Z component that contains most of the compressional seismic energy. Each trace shows its CABLE LENGTH header (i.e. the recording depth relative to KB) and the ACQUISITION SHOT NUMBER. The time scale of the plots is 20 cm/s, the traces are displayed equidistantly. Each second value of the cable length is displayed for plotting conveniences. 4 Time picking on the Hydrophone and Trace selection Channels created by the logging unit: Reference hydrophone 1: S2 (surface waveform data cluster) Downhole Geophone/GAC: X-Axis D1 (downhole 1 waveform data cluster) Y-Axis D2 (downhole 2 waveform data cluster) Z-Axis D3 (downhole 3 waveform data cluster) The arrival time was measured on individual traces of the surface hydrophone traces S2. The computation is made with an automatic algorithm that identifies the energy break at the first significant inflection point of the trace. The computed times were checked by the analyst and did not need to be manually corrected. The surface hydrophone times have been used as a timing reference (time zero) for stacking the downhole recorded waveforms. The downhole data are gathered in distinctive data files for the three orthogonal components of the tool accelerometer sensors) and each file is internally organized with increasing trace depths. The vertical component D3 is oriented along the tool main axis and hence follows the well deviation. As the maximum well deviation is near vertical, the D3 component comprises most of the energy of the direct compressional wave. Only one level(1600 m) has been rejected due to unreliable Transit times, and because it seems affected by casing ringing (located close to casing shoe). 5

6 5 Stacking, Timepicking and Deviation Corrections The shots of the three-downhole components have been referenced to the surface hydrophone transit time and summed with a median stack algorithm. The operation of stacking improves the signal to noise ratio and produces a single trace per component at each depth level. Given the relative positions of source and receivers in the 3D space and assuming a homogeneous subsurface medium, the observed transit times have been verticalized and referenced to the mean sea level. The amplitudes are normalized trace by trace and the S.E.G. reverse polarity is adopted. (A downgoing compressional wave is displayed as a white trough and recorded as a negative number on tape). 6 Remarks and Conclusions The Checkshot survey is acquired with the single level CSI tool that follows the well deviation profile and a gun array source located close to the wellhead. The first arrival compressional energy is essentially recorded on the tool Z-axis with good quality seismic waveforms. Only one level(1600 m) has been rejected due to unreliable transit times, and because it seems affected by casing ringing (located close to casing shoe). The inflection point transit times have been corrected for raypath obliquity and referenced to sea level to obtain the correct time-depth relationship in this well. 7 Results CD-ROM The results are stored on a CDROM containing SEG-Y files of recorded and stacked waveforms The present report in Word format The tables in appendix A in Excel format CGM file of wide scale display. The CD-ROM is labeled: EW

7 SHOT QC/ HYDROPHONE STATIC Fig 1 and 2 MEDIAN STACK CORRECT OBSERVED TRANSIT TIMES TO VERTICAL TRANSIT TIMES Apply deviation correction and determine geometry: replacement velocity =1524 m/s KB elevation = 43.5 m Source elevation 4 m below MSL, Hydrophone 8 m below MSL Final Display Fig 3,4,5 7

8 APPENDIX A Geogram Well Seismic report Excel spreadsheet with checkshot survey 8

9 APPENDIX B Displays Figure 1 Figure 1 shows the raw hydrophone data. All shots are displayed. Horizontally, the Cable Length and Acquisition Shot Number are given to correlate with the raw downhole receiver data (figure 2). Trace spacing is equi-distant. Amplitudes are normalized trace by trace. The observed travel times are both displayed numerically and graphically. Figure 2 Figures 2 show the raw geophone data from the tool component Z, oriented along the well axis. All traces are shown. Displayed Cable Length and Acquisition Shot Number are shown to correlate with the hydrophone signals. Trace spacing is equi-distant. Amplitudes are normalized trace by trace. Time zero is the firing instant. Final 3, 4, 5 The three plots show the stacked waveforms of the tool X component(fig 3), the Y component (fig 4) and the Z component (fig 5) and the picked transit times. The figures show traces after static correction with vertical transit time referenced to sea level. Amplitudes are normalized trace by trace. 9