Estimation of Oil-Water Contact Level Using Different Approaches: A Case Study for an Iraqi Carbonate Reservoir

In petroleum industry, an accurate description and estimation of the Oil-Water Contact (OWC) is very important in quantifying the resources (i.e. original oil in place (OIIP)), and optimizing production techniques, rates and overall management of the reservoir. Thus, OWC accurate estimation is crucial step for optimum reservoir characterization and exploration. This paper presents a comparison of three different methods (i.e. open hole well logging, MDT test and capillary pressure drainage data) to determine the oil water contact of a carbonate reservoir (Main Mishrif) in an Iraqi oil field "BG”. A total of three wells from "BG" oil field were evaluated by using interactive petrophysics software "IP v3.6". The results show that using the well logging interpretations leads to predict OWC depth of -3881 mssl. However, it shows variance in the estimated depth (WELL X; -3939, WELL Y; -3844, WELL Z; -3860) mssl, which is considered as an acceptable variation range due to the fact that OWC height level in reality is not constant and its elevation is usually changed laterally due to the complicated heterogeneity nature of the reservoirs. Furthermore, the results indicate that the MDT test can predict a depth of OWC at -3889 mssl, while the capillary drainage data results in a OWC depth of -3879 mssl. The proper MDT data and SCAL data are necessary to reduce the uncertainty in the estimation process. Accordingly, the best approach for estimating OWC is the combination of MDT No.27(6) 2020 Journal of Petroleum Research & Studies (JPRS) E96 and capillary pressure due to the field data obtained are more reliable than open hole well logs with many measurement uncertainties due to the fact of frequent borehole conditions.

wells from "BG" oil field were evaluated by using interactive petrophysics software "IP v3.6". The results show that using the well logging interpretations leads to predict OWC depth of -3881 mssl. However, it shows variance in the estimated depth (WELL X; -3939, WELL Y; -3844, WELL Z; -3860) mssl, which is considered as an acceptable variation range due to the fact that OWC height level in reality is not constant and its elevation is usually changed laterally due to the complicated heterogeneity nature of the reservoirs.
Furthermore, the results indicate that the MDT test can predict a depth of OWC at -3889 mssl, while the capillary drainage data results in a OWC depth of -3879 mssl. The proper MDT data and SCAL data are necessary to reduce the uncertainty in the estimation process. Accordingly, the best approach for estimating OWC is the combination of MDT No.27-(6) 2020 Journal of Petroleum Research & Studies (JPRS) E96 and capillary pressure due to the field data obtained are more reliable than open hole well logs with many measurement uncertainties due to the fact of frequent borehole conditions. Keywords: Oil

Introduction:
Oil-water contacts in a development wells usually are determined from water saturations derived from resistivity logs either by the detailed formation evaluation or by some quick look techniques. Unfortunately, well logs data is more frequently effected by many bad down hole conditions which give rise to erroneous in data acquisition and in turns less trusted and ambiguous interpretations [1,2]. As its importance in quantifying the hydrocarbon reserve, hence, it is essential to utilize different approaches to evaluate the OWC. An alternative and accurate method using formation tester and in combination with capillary pressure data can be used to validate the estimation process [3]. The proper data used in determination of OWC are given by the formation pressure testing tools such as modular dynamics tester (MDT) to measure formation pressure surveys through reservoir intervals [4]. When adequate data can be collected, the fluid contacts is determined very accurately by identifying the depths at the characteristic pressure gradients change [5].
However, variations in the OWC are common from well to well due to differences in petrophysical properties of the formation (reservoir heterogeneity). In practice, an average value of the OWC is used in reserve estimation when volumetric methods are used. [6] Field Background: "BG" oilfield is located in the southeastern Iraq close to the Iraq-Iran border as shown in Figure (1). Structurally, "BG" oilfield ranges about (40km * 7km) with two domes in the north and south respectively, the south dome is shallower and covers bigger area. "BG" oilfield has two sets of reservoirs, Tertiary Asmari and Cretaceous Mishrif. 7 pay zones are divided in the Mishrif reservoir, which is MA, MB11, MB12, MB21, MB22, MC1 and MC2. The main pay zone is distributed in lower part of Mishrif reservoir. The main pay zone MB21 of Mishrif oil reservoir in "BG" oilfield has an oil-water system and is an edge water structure stratigraphic reservoir with wide oil-water transition zone. The pay zones of MC1 are also an edge water structure stratigraphic reservoir. The natural energy in Mishrif oil reservoir of the oilfield is weaker than that in Asmari reservoir of Abu Ghirab oilfield and Asmari reservoir of Fauqi oil field but is stronger than that in Mishrif reservoir of Fauqi oilfield. "BG" oilfield was put into production in November 1976 and are produced from the Mishrif reservoir with regular well pattern and large well spacing ( 800 m). The production rate reached 40kbbls/d before it was shut down for more than ten years during 1980-1998 due to the Iraq-Iran war. After the oilfields resumes production in 1998, it has maintained the production level at about 35kbbls/d. Pay zone MB21 contribute 95% oil

Methodology:
The main steps for achieving this work can be summarized as shown below: 1-Collect open hole logs data, special core analysis (SCAL) and MDT pressure data for X, Y and Z wells.
2-Import the LAS files of log data for each well into Interactive Petrophysics (IP 3.6) software.
3-Estimate shale volume for each digit log interval for each well by using raw log data of gamma ray based on old rock module (Larionov).

(5)
Sonic porosity is used in wash out intervals in combination with Neutron porosity using the following equations: Therefore, data of available two wells (WELL X and WELL Y) were collected and quality checked for supercharges and bad tool readings and some reading points has been excluded from the interpretations.
Eventually, a universal capillary pressure (Sw vs height) called J-Leverett function has been derived for the entire formation from the available data of special core analysis of well BU-3 therefore the capillary pressure data was converted to reservoir conditions then J Leverett function has been calculated to normalize the variation in the petrophysical core properties (K&phi). J (sw) curves then the normalized J (Sw) curve has been converted to PC and the latter used to derive the saturation depth relationship and the corresponded Sw have been plotted on Cartesian graph for each core on same plot with different depth in MB21 reservoir. A universal J function has been derived with best fit to J (sw).
Finally, the following equations can be applied to estimate OWC based on the universal saturation-height relationship, Pd value, height above FWL, and FWL value:

Results & Discussion:
Firstly, the results of Picket plot for all three candidate wells presents that default Archie parameters (a=1, m=2, n=2) for limestone formations and Rw value equal to 0.02 from lab analysis are consistent with ones derived from Picket plot as presented in Figures (2) to (4).

Fig. (4) WELL Z Picket plot
The well correlation between these three candidate wells in this work is presented in the  Noticeably, it can be inferred from the interpretation results that no adequate MDT data has been measured since all pressure tests are conducted only in the oil zone of MB21 and no test points where performed in the water column . Therefore, it was impossible to predict the free water level with these available data. For Well-X, the interpretation results present an oil density of 0.85 gm/cc which is belong to just the oil zone and no measurements extended below the oil interval as shown in Figure (

Conclusions:
1) OWC estimation from well logs interpretations introduced a noticeable variance due to the reservoir heterogeneity that gives rise to conclusion that more wells from different regions of the reservoir are needed to be evaluated to get an accurate value of the predicted OWC for whole reservoir.
2) The results of OWC prediction by using open hole log interpretations method presented a good agreement with that approach based on capillary pressure data.
3) The MDT pressure test data are failed to estimate the FWL depth due to the insufficient pressure measurements in the field under study, where all the pressure test points are carried out in the oil column only.

4)
Adequate MDT data and more SCAL data are necessary to minimize the uncertainties in the OWC estimation process.
5) The best approach for estimating OWC is the combination of using capillary pressure and MDT data as a result of the data acquired are more dependable than open hole well logs which involved many measurement uncertainties due to the fact of frequent borehole conditions.