Field Development Project 5 – Estimates of Stoiip and Recoverable Resources, including Associated Uncertainty PDF

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Estimating the original oil in place for the Wytch Farm Field Development Project....

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Field Development Project 5 – Estimates of STOIIP and Recoverable Resources, including Associated Uncertainty

Key parameters in STOIIP Estimate and associated uncertainty

Stock Tank Oil Initially in Place, which is known as STOIIP, can be influenced by the gross rock volume (GRV), net to gross (NTG), oil volume factor (Bo), porosity and water saturation (Sw). The values of these parameters have different sources such as seismic, wireline logging and PVT analysis. The uncertainties included in the process of data output should be taken into consideration while analyzing the STOIIP uncertainties.

Gross Rock Volume – High Uncertainty

Gross rock volume is the volume between the top reservoir and oil-water contact (OWC). It is influenced by the top reservoir, oil-water contact and the velocity model used in seismic interpretation. •

For the top reservoir, because of the depth-related quality loss during seismic interpretation, there is no clear brightness of the reflector between Mercia Mudstone and Sherwood.

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Top and base Sherwood were picked using a bulk-shifted Mercia Mudstone as a guideline, while locally following the reflector trends, which means the same thickness variation of Mercia was used while building up the top and base of the reservoir.

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The variation of the top reservoir can lead to a significant change in GRV.

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The OWC was set during wireline logging interpretation using the resistivity difference of oil and water.

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Because the variation of the data is messy and the division of oil and water cannot be precisely identified, with the errors of around ±10 m, which can result in the uncertainty of GRV.

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In addition, oil-water contact can be also obtained from Dean-Stark Analysis, the related uncertainties of which need to be taken into consideration.

Because all horizons in seismic interpretation are in time units, so the appropriate velocity model is needed to transfer the time into distance. The constant model with the velocity of 3880 m/s in Sherwood was chosen for the conversion, which can lead to huge uncertainty. This is because of the different layers inside the reservoir in which the velocity is not constant.

Net to Gross – High Uncertainty

During the wireline interpretation, the histogram was used to obtain the value of clay volume cutoff by dividing the data distribution into two distinct groups. The sand volume will shift with the movement of the division line, which results in the high uncertainty of NTG.

Porosity – Low Uncertainty

Porosity values can be obtained from both wireline logging and core data. It is a direct parameter used in STOIIP equation as well as an input of the water saturation formula, so it can be regarded as a parameter with a double-effect on STOIIP estimation. For the porosity from wireline logging, it is obtained using a neutron-density cross plot with an uncertainty of the position of the shale line. For the core data, the uncertainty can be related to the core selection, handling process and experiments process of porosity measurement.

Bo – Low Uncertainty

The oil volume factor is calculated by combining the differential liberation and flash tests which were corrected with Bo at bubble pressure using Dake’s correction formula. During the test, only one sample from one single well was selected, which will present some uncertainty because of the lack of quality check and the fluid property variation within the reservoir.

Sw – Medium Uncertainty

The water saturation used in STOIIP calculation is from J function, which takes permeability, porosity, free water level and irreducible water saturation into consideration.

The correlation of porosity and permeability can be obtained using the core data from two wells, which can be uncertain because the correlation formula only matches a small part of the data. And the free water level from the RFT data is not the same in different wells and the reservoir condition is not static, which can be regarded as another uncertain factor. In addition, the irreducible water saturation measurement was done in the laboratory, and although the experiment was converted to reservoir condition, there will be some uncertainties with the degree of representativeness, and the quality of the plugs also has to be taken into consideration.

Key uncertainties assessment

To get the sensitivity of different uncertain parameters in STOIIP calculation, the sensitivity model was run using the distribution and range listed in table 1. Fig.1 is the Tornado chart showing the range of STOIIP values for the uncertain parameters in the sensitivity analysis.

Variable Oil Water Contact Sand Content Zone 1 Sand Content Zone 2 Sand Content Zone 3

Distribution Triangular Triangular Triangular Triangular

Maximum Porosity Zone 1

Truncated Normal

Maximum Porosity Zone 2

Truncated Normal

Maximum Porosity Zone 3

Truncated Normal

Minimum Porosity Zone 2

Truncated Normal

Minimum Porosity Zone 3

Truncated Normal

Maximum Permeability Zone 1 Maximum Permeability Zone 2 Maximum Permeability Zone 3 Minimum Permeability Zone 1 Minimum Permeability Zone 2

Truncated Log-normal Truncated Log-normal Truncated Log-normal

Range of Value Min = -1665, Max = -1632, Mode = -1642 Min = 50, Max = 80, Mode = 60 Min = 70, Max = 99, Mode = 85 Min = 70, Max = 99, Mode = 85 Min = 0.25, Max = 0.5, Mean = 0.3, Std. = 0.05 Min = 0.26, Max = 0.5, Mean = 0.31, Std. = 0.05 Min = 0.28, Max = 0.5, Mean = 0.32, Std. = 0.05 Min = 0.01, Max = 0.06, Mean = 0.03, Std. = 0.05 Min = 0.005, Max = 0.03, Mean = 0.01, Std. = 0.05 Min = 0, Max = 10000, Mean = 102.57, Std. = 141.12 Min = 0, Max = 10000, Mean = 118.06, Std. = 222.34 Min = 0, Max = 10000, Mean = 281.42, Std. = 512.27

Truncated Log-normal

Min = 0, Max = 20, Mean = 5, Std. = 5

Truncated Log-normal

Min = 0, Max = 20, Mean = 5, Std. = 5

Minimum Permeability Zone 3 Bo

Truncated Log-normal

Min = 0, Max = 20, Mean = 5, Std. = 5

Triangular

Min = 1.2, Max = 1.22, Mode = 1.21

Channel Width Zone 2

Truncated Normal

Channel Width Zone 3

Truncated Log-normal

NTG Irreducible Water

Triangular

300 Min = 100, Max = 3500, Mean = 800, Std. = 400 Min = 0.96, Max = 0.98, Mode = 0.97

Triangular

Tornado Chart

Min = 0.15, Max = 0.25, Mode = 0.2

From the Tornado Chart, the most sensitive parameter in STOIIP calculation is oil water contact, which is followed by sand content in zone 2, irreducible water saturation and channel width in zone 2. Except the parameters shown on the Tornado Chart, the STOIIP value is also highly sensitive to the top of Sherwood. They are not included in the Tornado Chart because they do not change in an entire scale. The depth of the top and bottom of the five wells should remain constant during the sensitivity model simulation. As a result, the standard deviation of 20 m is set between the wells to simulate the uncertainty of top and base of the reservoir.

Recovery Factor

Analogue basins with geology and reservoir characteristics which are similar to the Wytch Farm Oilfield were studied to estimate the likely recovery factor. Technical parameters of Wytch Farm Oilfield had been closely compared to the parameters of the existing oilfield and an average value of the recovery factor was estimated from Brent, Statfjord, Asgard, Snorre-Lunde and Judy-Skagerrak oilfields because of their similar properties with Wytch Farm. There will be some uncertainties existing during the process of searching for global analogues because of the enormous database. Most recovery mechanism used in the analogues is water injection, and this is because the aquifer is not strong enough for primary oil production (same with the Wytch Farm Oilfield). For Sherwood, aquifer drive and water injection are the primary and secondary recovery mechanism respectively. Monte Carlo simulation has been applied to the range from 0.27 to 0.66 with a rectangular distribution. The result of the recoverable reserves is shown in Fig. 3.

STOIIP and Reserves

The P10, P50 and P90 values for STOIIP and estimated ultimate recovery of oil and gas are shown in Table 2. Based on a gas-oil ratio (GOR) value from PVT analysis, the amount of gas in place and the proportion of gas that would appear at surface conditions had to be taken into consideration.

The results show large volume of gas being recovered which will have to be accounted for during recovery either it will be produced or reinjected to prolong the reservoir life.

Histogram for STOIIP

Histogram for Recoverable Reserves (oil)

Histogram for Recoverable Reserves (gas)

Result of STOIIP and Estimated Ultimate Recovery

STOIIP EUR (oil) Unit EUR (gas)

P90 MMSTB 655 340 Tcf 1.2

P50 MMSTB 786 415 Tcf 1.4

P10 MMSTB 969 503 Tcf 1.7...