One of the specific features of Brownfield development is a gradual yet steady production decline. First and foremost, this is caused by reserves depletion, reservoir pressure decline, and increased watercut. Various enhanced oil recovery (EOR) techniques are used to maintain production or slow down its declining production rates and therefore develop the field more efficiently and increase ultimate oil recovery.
EOR performance evaluation relies on the estimation of post-well workover incremental oil production. It can be determined through a number of techniques, all
of which compare base (forecasted) oil production to actual oil production by the end of the calculation period. It is this difference that shows the incremental oil production volume.
To determine base production, displacement characteristics are used which most accurately correspond to the current development conditions, i.e. are most similar to those of the target. On the other hand, EOR performance evaluation considers production forecasts for the coming period, since well workover performance is also planned within this process. In other words, oil production planning or incremental production planning should use the same processes or procedures with the same calculation principles as those used for well workover performance evaluation.
The currently used method to evaluate well workover (EOR) performance is based on the oil production forecast procedure developed by TNK-BP specialists back in 2007. According to this procedure, oil production forecasting should consider oil losses caused by the development target depletion and watercut increases as well as well decommissioning. Oil losses are calculated by the exponential relationship that relies on the production decline for the coming period, which in its turn is determined based on the performance in the previous period. Losses are forecasted more accurately when production decline rates are calculated independently for the base well-stock where no well workover (EOR) was performed and the well-stock subjected to well workover (EOR); in the latter case, calculations should be made for each well workover (EOR) type individually since production decline rates can differ significantly.
Current Method to Evaluate Well workover Performance
The following algorithm to calculate incremental oil production is currently used to evaluate well workover (EOR) performance.
First, the production decline exponential factor is determined for the base well-stock. It includes all wells in the field regardless of their location and the reservoirs operated where no well workover (EOR) was performed in the previous period. Daily production rates of all wells in the first and the twelfth month of the year are used to calculate the production decline exponential factor:
» qn is production in the beginning of the calculation period (month 1)
» qk is production in the end of the calculation period (month 12)
The same formula is used to calculate production decline exponential factor for wells after well workover (EOR). The calculation considers data from all wells where well workover was performed under similar conditions, regardless of their location in the field and reservoirs operated.
After that base daily production and post-EOR daily production are forecasted for 12 months with due regard for the production decline rate in the previous period. For wells after EOR, flow rate with baseline increment serves as the initial flow rate. Finally, base cumulative production forecast is compared to post-EOR cumulative production forecast with due regard for well operation time during the calculation period. The difference between the two will be the annual incremental production.
Thus, the production decline rate calculated by the exponential relationship (I) is the key determinant to forecast incremental oil production and evaluate well workover performance.
Take a look at the daily rate profiles and production decline exponential factors by target (Table 1) and development element (Table 2). The significant variations are caused by different geological and physical characteristics of reservoirs (targets), different physical and chemical properties of produced fluid as well as by different reservoir energy conditions, compensation ratios, etc.
This means that incremental production after well workover (EOR) can be calculated incorrectly if the base well-stock and post-EOR well-stock operate different targets. In other words, well workover performance evaluation should rely on production decline rates calculated individually for each target or even each development element wherever possible.
Each Development Element Treated Individually
The new method to evaluate well workover performance also determines decline rates for base production and post-EOR production, yet considers wells operating the same development target or the same development element. Let us evaluate performance of two types of well workover, i.e. optimization and bottomhole treatment (BHT) while well servicing, performed in wells of M-1 element of Samotlor’s BV10(1-2) reservoir (Fig. 1).
First, wells are identified that operate the selected target and experienced no well workover (EOR) in the previous period (one year); in our case, these were wells 800е, 815е, 801е, 888е, and 845е. The average flow rate of the selected wells is determined for months 1 and 12, and formula (I) is used to calculate base oil production decline rate. For M-1 element of BV10(1-2) reservoir the exponential factor of base production decline equaled to 93.2 percent (Table 3).
After that, wells are selected that operate the same development target and experienced a specific EOR type in the previous period (one year). In our case, wells 806е and 814е were subjected to optimization and wells 881е and 843е were subjected to BHT well servicing. For these wells, the average flow rate for months 1 and 12 is also determined, and the production decline rate is also calculated by formula (I). Actual data from wells 806е, 814е, 881е, and 843е were used to derive production decline exponential factors after optimization and BHT while well servicing and these amounted to 79.6 percent and 67.1 percent, correspondingly (Table 4).
To forecast base production, base well-stock production decline rate in the pervious period (Table 3) is used; the initial flow rate in this case is the actual well flow rate as of the EOR application date. Similar principle is applied to forecast post-EOR production. In this case, production decline rates by EOR types are used (Table 4) while the expected post-EOR flow rate (+5 t after optimization and +3 t after BHT) is used as the initial flow rate. Thus, the forecasted EOR effect was 1,395 t after optimization and 947 t after BHT while well servicing (Table 5).
The principle to define well workover actual performance is similar to that to forecast performance; the only difference is that it is the post-EOR flow rate in the first month of operation that is taken as the initial flow rate. The margin between cumulative base production forecast obtained with due regard for the production decline rate and actual cumulative oil production by the end of the calculation period is actually the post-EOR incremental oil production. In our example, incremental oil production reached 2,052 t after optimization and 179 t after BHT while well servicing (Table 6, Fig. 2).
It is obvious that in the current example the difference between the production forecast and the actual production is significant; this is explained by insufficient number of wells selected to establish production decline rates as well as reservoir pressure maintenance impact. However, the described restriction to the current method to evaluate well workover (EOR) performance contributes to the accuracy of incremental oil production determination and in some cases can even increase incremental production due to a more accurate base production estimation. Besides, with this restriction in place well interference is taken into account, since its influence will be more distinct while analyzing wells of one and the same development element.
Published with thanks to TNK-BP and Innovator Magazine