Appraising and developing STX field : determination of uncertainties by DST analysis

A subsurface uncertainties is a possible future event, which, if occurs, would affect project objectives either negatively or positively. For any given model or event, the uncertainty is the range of variation of the component parts and possible outcomes. It could be quantified approximately by either analytical model or in a more cumbersome one such as numerical approach. This paper summarizes thedetermination ofuncertainties by DST analysis in appraising and developing the ST-X gas condensate field, which is offshore Vietnam in Block 15-1O. Drill Stem Test (DST) results show that the S field has moderate to low permeability, multiple flow boundaries/barriers, and at least 2 PVT regions. To understand the impact of these and other important reservoir parameters on ultimate gas and condensate recovery and well count, the uncertainties has to be well evaluated and understood. The study demonstrates that there is a wide range of possible ultimate gas and condensate recoveries and well counts. The top causes for this wide range are permeability and flow boundaries/barriers. In addition to the subsurface risks, drilling cost of a ST-X well is very high. The high well cost in combination with the field being offshore, having low permeability and possibly numerous reservoir compartments dramatically increase the risk of a full field development. Key word: uncertainty analysis, well test analysis, history matching, sensitivity analysis.


INTRODUCTION
The ST-X field is in the Cuu Long basin with approximately 155 km east of Vung Tau, 62 km offshore Vietnam, in 66 meters of water (Figure 1).Four wells have been drilled in the ST-X field to date (Figure 2).Well-Bwas drilled to evaluate the faulted and fractured basement reservoir, as well as, the Oligocene sandstones sequences.The second appraisal well,Well-Cwas drilled to evaluate the down flank extent of the sand sequences and an untested fault block.
The Well-Dwell was drilled to test the Oligocene clastics on the northern flank of the ST-X structure.

Reservoir Uncertainties
Static reservoir properties are such as Net Sand, Sand Porosity and Oil Saturation.This includes the uncertainty in petrophysical derivation of well-logs, plus the lateral distribution of the static reservoir properties across the reservoir (controlled by the depositional facies scenario).The major impact of Static Reservoir properties uncertainty is on STOIIP and the reserve output.Permeability, cross plotting of the porosity and permeability data derived from core, well test, mini-DST, and MDT/RCI indicates scope for alternative regression lines to fitted through this data.Theoretically, two main categories of uncertainties that can potentially impact the value of the field development -Static Uncertainties mainly impacting STOIIP (from structural, depositional and fluid contact uncertainty) -Dynamic Uncertainties impacting long term reservoir sweep and productivity.These categories of uncertainties combined describe a range of ultimate recoveries and production forecasts.

Drill Stem Testing (DST)
Well testing has progressed to become one of the most powerful tools for determining complex reservoir characteristics.It emphasizes the need for both a controlled downhole environment and high-performance gauges, which have made well testing a powerful reservoir description tool.Generally the Well Testing Interpretation results are: -The reservoir production capacity (transmissibility) -The well production capacity (well damage) -The reservoir limits (reservoir porous volume) -The reservoir specific behaviors During a well test, a particular flow rate history is applied to a well, and the resulting pressure changes are recorded, either in the same well (typically) or in a nearby well interference test.From the measured pressure response, and from predictions of how reservoir properties influence that response, an insight can be gained into those reservoirs properties.In order to make these predictions, it is necessary to develop mathematical models of the physical behavior taking place in the reservoir.
In view of modeling, good quality DST data promises bringing reliable dynamic modeling result.Condition is that the calibration approach shall be reasonable to capture the variation in reservoir property with no over or under its estimation potential.A systematic approach of using dynamic model to assess the variation of well test interpretation result to the range of output recovery factor as depicted in Figure 3.

Methodology
Analysis and evaluation of uncertain factors include three basic steps: identification of uncertain factors, determined domain of uncertain factors and screening uncertain factors.Within the scope of this study, step 1 in the process of defining the elements are unlikely to be present.In particular, the uncertainty factors are identified through interpretation of dynamic data during testing.These factors include : K, Skin, Tran, Fluid, Boundary, Condensate blockage, Porosity, Fault, absolute permeability, rock compression ... Based on the uncertainty factors have been identified , the suspect may affect the model simulation results.These uncertainties may be related to geological and technological factors as discussed above.These factors have been the strongest impact on model outputs.These factors are selected based on the characteristics of each reservoir, as well as on the experience of the engineer.The determination of value domain must be consulted by the experts of geology and reservoir engineering.
Besides, the methodology has been based upon reservoir simulation predictions using the available simulation models which have been calibrated to DST data.The reasonable case sensitivities have been performed through variation of various parameters including OIIP changes, well counts and static & dynamic properties.
The work flow for dynamic modeling work is essential in the sense that it allows a systematic approach for any modeling work.
Two major groups in the process includes DST calibration such that the model will be tuned to testing data to a certain confident level, then the well placement steps ensure capturing potential productive areas, determine optimum number of well as well as its trajectory, perforation policy and so on.The last step in the process is to analyze and sort out the uncertainty factor in the Tornado chart prior to come up with a final recovery factors.

Appraisal wells results
DST's wereconductedon the Well-A(D, E and F Sand); Well-B(Basement); Well-C(E and F Sand) and Well-D(E sand)wells.Table 1 summarizes the flow properties determined from these tests for each well and sand sequence.In addition to the PVT data obtained from the DST's (Table 2), MDT data also provides an understanding of how the PVT properties may vary within the reservoir (Figure 4).They indicate that potentially three PVT regimes may exist in the field.

DETERMINATION OF UNCERTAINTIES BY DST ANALYSIS
Derivative analysis was performed on the initial build up, main flow period and main build up for all well of ST Field.For simplicity, only the gas rates and bottom hole pressure have been input into the analysis.Pressure analysis was performed using the following set of input data as below.Derivative analysis was performed on the main build up period.This derivative is shown in Figure 6 and 7: the log -log plot and semilog plotof the final build up with single layer model.By matching this plot, derivative pressure curve of this DST indicates a radial flow period followed by a period that appears to be effected by boundaries.However, late time period of derivative curve still has been no good matching due to single layer is only sensitive with boundary close to the well.The simplest solution that is able to achieve satisfactory matches on both the derivative and the full flowing period is shown above.This is a radial composite system with parallel faults at 675 feet and 44 feet from the well.Permeability in the well is somewhat uncertain due to the uncertainty in picking radial flow.By matching this plot and attempting to match the full history an attempt at arriving at values for kh, Skin and Cs can be made.

CONCLUSIONS AND RECOMMENDATIONS
Theresults of this work show that there remains significant reservoir uncertainties in the ST-X field and thesimulated recovery factor can vary greatly.The well count forthe good reservoir permeability and connectivity scenario is much lower than for the case where the reservoir has poor permeability and connectivity.Additionally, during the exploration and appraisal phase of the ST-X field, it was found that the drilling cost of a ST-X wells are very high.The high drilling cost combined with the field being offshore and the reservoir having both low permeability and potentially large numbers of reservoir flow boundaries make a full field development a high risk endeavor.
For these reasons an Early Production Systemis recommended to reduce the development risk.In addition to generating revenue by selling the produced condensate and gas, the production data will improve the understanding of the field'spermeability distribution and connectivity.The reservoir information obtained from the Early Production System will be vital input for further consideration of a full field development plan ofST-X Field.

Figure 3 .
Figure 3. Well Testing is Indispensable part of Reservoir Description and Management

Figure 5 . 3 Figure 6 .Figure 7 .
Figure 5. Gas Rate and Pressure for Analysis in DST#3 This pressure behavior suggests that two boundaries were encountered.A good match to Trang 32 the boundary effects can be obtained by change multi layer and boundary model (parallel faults).

Figure 8 .Figure 9 .
Figure 8. Log-Log plot of the final build up(three layer with parallel boundary)

Table 1 .
Flow Properties Seen on DST's