Drilling exploration wells always carry increased risk given the lack of offset wells. Once of the main uncertainties usually is the depth prognosis derived from surface seismic depth converted data. The accurate prediction of the depth of top carbonate while drilling carbonate reservoirs is important to avoid losses and well control problems. This can have a significant financial impact. This paper demonstrates the use of innovative look-ahead VSP technology in Vietnam to refine depth prediction of the top of carbonate ahead of the bit for accurate geo-stopping. An offshore exploration well was being drilled in an area with drilling challenges. This was the first well in the field and lacked offset well velocity information, so there was a high degree of depth uncertainty to the top of the carbonate target from surface seismic interpretations, making drilling operations risky. An intermediate VSP was requested to look for reflections ahead of the intermediate TD using Schlumberger VSI1 tool. The VSP data was acquired in open and cased hole using an air gun cluster as the seismic source. The drilling program required the casing to be set immediately above the top of the carbonate. By acquiring an intermediate VSP, the depth uncertainty was reduced and the target depth prediction was refined. This was achieved using the look-ahead VSP technique, from which the acoustic impedance and velocity ahead of the bit was estimated. This was combined with time-depth from the VSP data over the logged interval to help refine the predicted depth to the top of the carbonate. This was used to optimize the depth at which the casing was set. The depth uncertainty of the key formation top (top of carbonate reservoir) from the surface seismic interpretation was over 180m prior to drilling. The final prediction depth from the VSP interpretation is 7m shallower than the actual carbonate reservoir. Guided by the inverted acoustic impedance log and velocity ahead of the bit from look-ahead VSP, a further 150m was safely drilled before setting the casing. The casing point was set successfully without penetrating the carbonate. The intermediate VSP results helped to decide drilling ahead in a safe manner with confidence, after setting the casing before penetrating the carbonate target.

The Vertical Seismic Profile (VSP) technique is routinely used to create seismic images near the wellbore and traditionally only the up-going wavefield is used to image the subsurface below the receivers. The standard VSP technique does not give any information of the seismic image above the well trajectory. The objective here is to produce a seismic image above the top receiver depth up to sea floor using the downgoing multiples recorded in the VSP for a more complete seismic to well-tie. A seismic source is positioned below Mean Sea Level and deployed near the wellhead. The source signal is recorded by a downhole receiver that is moved to cover a large number of depth levels in the well. The upgoing and downgoing arrivals are separated during processing; the up-going wavefield is used for subsurface illumination while the down- going wavefield and multiples are normally excluded from the processing. The standard VSP technique using VSP upgoing wavefield gives a seismic image along the range of receiver depths and below the well trajectory. However, a VSP image can also be obtained from the downgoing multiple sequences. The main application of processing the sea surface multiple is to obtain a VSP image of formations above the top receiver depth which is unattainable with standard VSP technique. Results show that by using multiples, illumination coverage increases significantly in comparison to using primaries. In addition, reflectors above the shallowest receiver can be imaged by multiples, including the sea bed itself. This can be useful for different purposes such as shallow hazard identification for sidetracks or to avoid the expense of in-fill nodal seismic below the rig. We also show that the image obtained from the VSP multiples can have good resolution comparing to the surface seismic data, with better signal to noise ratio especially for the shallower reflectors. The VSP data is true amplitude, zero-phase and perfectly tied in depth and in time. By using the sea surface related VSP multiples and the mirror imaging technique, the VSP image was extended successfully above the well trajectory up to the sea floor and shows a good correlation with the surface seismic. In this case study, we present some results of using the sea surface downgoing multiple for this unconventional VSP technique.

No abstract

The accurate prediction of the depth of top carbonate while drilling carbonate reservoirs is important to avoid losses and well control problems. This can have a significant financial impact. This paper demonstrates the use of innovative look-ahead VSP technology in Vietnam to refine depth prediction of the top of carbonate ahead of the bit for accurate geo-stopping. An offshore exploration well was being drilled in an area with drilling challenges. This was the first well in the field and lacked offset well velocity information, so there was a high degree of depth uncertainty to the top of the carbonate target from surface seismic interpretations, making drilling operations risky. An intermediate VSP was requested to look for reflections ahead of the intermediate TD using Schlumberger VSI1 tool. The VSP data was acquired in open and cased hole using an air gun cluster as the seismic source. The drilling program required the casing to be set immediately above the top of the carbonate. By acquiring an intermediate VSP, the depth uncertainty was reduced and the target depth prediction was refined. This was achieved using the look-ahead VSP technique, from which the acoustic impedance and velocity ahead of the bit was estimated. This was combined with time-depth from the VSP data over the logged interval to help refine the predicted depth to the top of the carbonate. This was used to optimize the depth at which the casing was set. The error range for the top carbonate depth from the surface seismic interpretation was +/−50m. The final prediction depth from the VSP interpretation is 6m shallower than the actual carbonate reservoir. Guided by the inverted acoustic impedance log and velocity ahead of the bit from look-ahead VSP, a further 100m was safely drilled before setting the casing. The casing point was set successfully without penetrating the carbonate. The intermediate VSP results helped to decide drilling ahead in a safe manner with confidence, after setting the casing before penetrating the carbonate target.