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Analysis of Dst to Predict Oil Sand: a Case Study of Kailashtilla Field

Mohammad Amirul Islam
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh

A.S.M. Woobaidullah
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh

Badrul Imam
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh

Abstract/ Executive Summary:

Drill Stem Test (DST) describes the dynamic characteristic of the hydrocarbon bearing formation such as wellbore storage, skin effect, permeability, average reservoir pressure and reservoir boundary. The wellbore storage effect and average reservoir pressure help to predict the flowing phase from the reservoir. An effort has been made to analyze the DST conducted in the Kailashtilla field at the depth interval 3261 meter to 3266 meter in well KTL-7. Two sets of pressure profile have been recorded. First conditioning the well for an hour then performed drawdown following pressure build-up. The pressure signature of the buildup period and its derivative is plotted on semi-log and log-log coordinates to develop Horner and diagnostic plots, respectively. Wellbore storage, skin and transient flow effects have been observed in the DST analysis which is an indication of the hydrocarbon bearing reservoir in the zone of interest. The value of wellbore storage effect is low which predicts the flow of liquid hydrocarbon into the well bore from the reservoir. Average pressure of the investigated zone has been estimated which is higher than the water column pressure. The higher average reservoir pressure also authenticates the presence of oil reservoir.

Keywords: Well log, DST, Pressure buildup, Wellbore storage, Average reservoir pressure

Location: Kailashtilla field, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Soil Physical Properties

Objectives:

To analyze the DST conducted in the Kailashtilla field at the depth interval 3261 meter to 3266 meter in well KTL-7.

Methodology:

The analysis of pressure signature obtained from the DST the wellbore storage, skin factor, permeability and average reservoir pressure have been estimated and their values are analyzed to obtain the following: 1. The flowing phase during the DST is liquid on an average rate 1000 bbl/d. 2. Although the reservoir permeability is low, the negative skin factor helps the reservoir liquid to flow into the wellbore. 3. The low value of wellbore storage provides evidence of the presence of liquid phase that has flown into the wellbore from the reservoir. There is no flow of gas phase into the wellbore from the reservoir. 4. Average reservoir pressure and water column pressure at depth 3266 m reveals the existence of the overpressure zone which is developed by the presence of hydrocarbon in the liquid phase. 5. Well, log analysis i.e. low value of gamma-ray log and high value of resistivity log with shallow and deep resistivity separation indicates the presence of hydrocarbon as well. From the above analysis, it can be concluded that all of the investigations i.e. well logs and DST analysis are able to provide evidence of the presence of liquid hydrocarbon (oil) in the interval 3261 to 3266 meter. The DST interval is selected on the basis of the well log analysis. In the interval 3261 meter to 3266 meter, the log analysis shows that low value of gamma ray log, high value of resistivity log with shallow and deep separation and high value of acoustic log indicating porous permeable formation with hydrocarbon bearing zone. To conduct the safe and proper DST operation it is very important to design the DST string and the Bottom Hole Assembly (BHA) according to the collapse load, burst load and shear failure. The DST string and BHA are shown where drill pipe, drill collar, crossover, pressure gauge are used. The DST operation commenced on 9th February 2015 at 18.00 hours and terminated on 12th February at 12.00 hours. During the total test period, first the well bore conditioning and pressure gauge calibration operation are performed in the initial hydrostatic period then started the drawdown period for 760 minutes following build-up period for 892 minutes. Four pressure gauge and temperature recorder have been installed in the test stem for recording four sets of data. Two record numbers 1785 and 40914 have shown the reservoir responses in the pressure profile which have been analyzed. A pressure gauge is set at the depth 3239.5 meter to record the flowing pressure during the DST operation and pressure signature is recorded under the record no. 1785. In the total pressure profile of the DST there is presence of drawdown following buildup pressure signature from 26.02 to 37.57 hrs and from 37.57 to 53.68 hrs, respectively since the start of test. The buildup pressure (ΔPbu = Pws – Pwf) and its derivative [dΔPbu/d(tp + Δt)/Δt] is plotted in log scale along the Horner time [(tp + Δt)/Δt] in the same scale to build the diagnostic plot where wellbore storage effect, skin effect and infinite acting reservoir responses are visible clearly. The well bore storage is 0.21bbl/psi and from the infinite acting line the permeability is 6.3312 md.

Source of Information: Bangladesh J. Sci. Res. 29(1): 19-29, 2016 (June)

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Knowledge Management: Journal