Appraisal Well Design in "X" Oil Field

The selection of casing depths and casing design are considered one of the most critical steps in the well construction process. Inaccurate selection of casing setting depths and casing design can result in many challenges, long time and hence high well costs. "X" oil field was taken as a case study. There is only the exploration well X-1 drilled up to date. The 20 ̋ surface casing was relatively long because it was set at the top of Dammam Formation. That means deep surface hole, long trip time, large amount of mud, long surface casing, large amount of cement and hence high cost. Also, Hartha Formation was not evaluated because it is isolated with the 13 3/8" & 9 5/8" casing and the perforation through two casing was not available. Another problem, the 9 5/8 ̋ production casing damaged at the depth 32 m due to failure of tolerating the axial forces before or after the cement job. All data was inputted into the Landmark software to simulate the well. It was found that the surface casing can be set at the top of Lower Faris Formation instead of Dammam Formation. Also, The Hartha Formation can be drilled in the 12 1⁄4  ̋ hole and isolated by 9 5/8 ̋ casing instead of drilling it in the 17 1⁄2  ̋ hole and isolating it by the 13 3/8 ̋ casing. It was also found that the 9 5/8 ̋ production casing can withstand all loads by selecting casing with higher weight. The cost of the modified design was also checked to study the feasibility of the modified design. It was concluded that the modified design can save around 300,000 USD for each well comparing with the design of well X-1. It is recommended to apply this design on the appraisal well to be drilled. If the design shows no problems, it can be considered the optimum design of appraisal and development wells to be drilled in the future. Also, the slim-hole design can be studied This work is licensed under a Creative Commons Attribution 4.0 International License. Journal of Petroleum Research and Studies PISSN: 2220-5381 EISSN: 2710-1096 Open Access No. 34 part 1, March 2022, pp.31-50 32 and an economic feasibility comparison can be made with the current and the proposed design in this study.


Introduction
The Hole problems such as lost circulation, pipe sticking and/or well control problems occur during drilling troublesome formations. Inaccurate selection of casing setting depths makes the problems worse and more difficult to cure. For example, if the surface hole is relatively deep, that means long trip time to change the bit, increase number of bits to use, large amounts of drilling mud, long casing string and large amounts of cement. That leads to increase the risk of encountering the drilling problems and to increase the overall well cost [1].
The challenges come from the drilling operations in the "X" oil field, a new onshore oil field. The surface casing string was set few meters in Dammam Formation. In this well, the surface casing was relatively deep which may cause many challenges to be encountered while drilling and evaluation stages. The 9 5/8˝ production casing had a damage problem which required work over operations. Hartha Formation is the last formation drilled in the 17 ½ " intermediate section in well X-1. Hartha Formation was isolated by 13 3/8" and 9 5/8" casing which made the evaluation of this formation difficult.
All challenges were studied in this research and solutions have been recommended to drill the next appraisal well Da-2. Also, the estimated cost reduction will be shown in comparison with the design of the exploration well X-1.

Casing Seat Selection
The selection of casing setting depths is considered the first step of the casing design process. Incorrect selection of casing setting depths can preclude the well from achieving its objective. Casing seat selection is governed by the following parameters: 1. Formation pore pressure.
2. Formation fracture pressure. 8. Regulations in which the field is located.
The casing setting depths are usually determined by two approaches:

Bottom-to-Top Approach
In this approach, the casing setting depths are selected by determining the depth of the production casing (or production liner), then the intermediate casing depth and after that the surface casing depth. The number of casing strings is governed by the depth of the well, the pore pressure gradient, the fracture pressure gradient, the kick tolerance and hole problems such as lost circulation, pipe sticking, wellbore stability. [3]

Top-to-Bottom Approach
The setting depths of casing in this approach are determined starting from the surface casing to the production casing or the production liner. That means the depth of the surface hole is determined first and then the intermediate and production hole sections respectively.[3]

Selection of Casing Sizes
Once the number of casing strings and their setting depths are determined, the size of each casing string should be determined. Typically, it is recommended to start determining the size of the last casing string to be set on the bottom of the well. The size of the last casing string depends on the type of completion to be used. In addition, the casing program should allow for alternatives in case an uncontrollable problem is encountered and an additional casing string is required to isolate the problematic interval. [1]

Casing Design Criteria
Once the number of casing strings, setting depths and casing sizes are determined, the next step is to design each casing string based on the expected loads acting on each casing during various operations and service life. Basically, three types of loads are considered as follows: 1. Collapse: Collapse loads are defined as the differential pressure in which the pressure outside the string exceeds the pressure inside the string. [3] Collapse pressure = P out -P in (1) Where: P out : pressure outside casing (psi).
P in : pressure inside casing (psi). Burst loads are defined as the differential pressure in which the pressure inside the string exceeds the pressure outside the string. [3] Burst pressure = P in -P out (2) 3. Axial loads: Axial loads causes tension or compression loads which mostly result from gravitational forces, frictional forces or changes in the pressure and temperature in the wellbore. In directional wells, there are also bending forces [4]. Also, bi-axial and tri-axial loads are taken into consideration.

Casing Specifications Selection
Once the maximum expected collapse and burst loadings are calculated and design lines are obtained, casing can be selected to approach the requirements of the design. Then, selected casing should be checked for axial, biaxial and tri-axial loads to ensure that it can withstand these loads during various stages. [5], [1] In addition to design criteria, many considerations contribute to the selection of casing. "X" oil field is a new oil field located in Missan/Iraq. The field is operated by Missan oil company (MOC). One vertical well (X-1) was drilled in the field in 2012 by the Iraqi drilling company.

Lithological Column and Geological Description
The lithological column and geological interpretation are illustrated in Table (1).

Well Sketch
The well sketch is shown in Figure (1).

Casing Setting Depths and Design Problems in Da-1
The problems, challenges and their outcomes are explained in the following points:      The 20" surface casing and the 13 3/8" intermediate casing setting depths have been changed by using the Landmark/ Casing seat as follows:

Proposed 20" Surface Casing Setting Depth
The main changes are explained as follows: 3. It should be mentioned that the 20˝ surface casing can be set at the same depth as in X-1 (at 1247 m) in case that Dammam is a thief zone. In case of no lost circulation is encountered, the 20˝ surface casing can be set at 750 m.
The design limitations are due to increase the hydrostatic pressure of mud with adding ECD, which can fracture the formation at some depth, and the limitations related to the influx volume (40 bbl as a worst case) require running casing. But, if the surface casing setting depth is less than 750 m, then the software will add one more casing string due to exceeding the maximum value of the kick tolerance which had been previously determined.

Proposed 13 3/8" Intermediate Casing Setting Depth
It was found that the casing seat can be at (+/-) 2000 m through Shiranish formation (Table 3). The latest formation is a competent formation which makes it suitable for setting the 13 3/8" intermediate casing string. When cementing the 13 3/8" casing, the most competent lithology must be selected for setting the casing and the casing must be raised around 5 m to be set at a strong cement structure. Due to changing the setting depth of the 13 3/8" intermediate casing, Hartha formation will be drilled in 12 ¼" the second intermediate hole section. In this case the cementing of the 9 5/8" casing that is run through the 12 ¼" hole must be two-stage cementing. The TOC of the first stage is at 1850 m. This will reduce the hydrostatic pressure that is exerted on Hartha formation.
Based on this scenario, the cementing of the 13 3/8" casing will be one stage instead of two-stage.

The 9 5/8" Production Casing Setting Depth
No change in setting depth is proposed, but the TOC should be at 538 m to tolerate the loads without failure. The length of the production casing can be either from the surface to the total depth or a production liner. In case of using a 7˝ production liner, the 9 5/8˝ will be considered as a production casing.  3. The other forces such as axial forces, tri-axial forces and the compression forces are considered after the casing selection in order to check the tolerance of the selected casing to those forces.
Based on the assumption mentioned in Figures (7 & 8), the casing specifications were determined as shown in Table ( the casing specification 20˝, K-55, 147 ppf, BTC will be the best option to use. d) The damage of the 9 5/8˝ production casing that occurred in X-1 can be avoided by selecting casing with higher weight. The proposed casing to be used is 9 5/8˝, 53.5 ppf, P-110, BTC instead of using 9 5/8˝, 47 ppf, P-110, BTC to avoid problems that may be encountered due to collapse and/or tension failure.

Economic Feasibility
The most important benefit obtained from the design modification must be the overall cost reduction resulting from consuming time and materials. The challenges of getting the well cost details of well X-1 led the research team to use the cost of other wells from another field in the same governorate "Missan". The cost data of the bits was neglected because it was assumed that the same type of bits used in X-1 will be used while drilling the next wells. Time saving is not easy to be estimated at this stage because the wells are either exploration wells or appraisal wells.

Casing Cost
The casing cost was got from MOC. Some casing grades or weights costs were not available, therefore the same cost of the closest casing grades and weights to the required casing grades and weights has been assumed. Table (5) shows the expected cost saving related the casing by applying the modified design.

Cementing Cost
The cementing cost data was obtained from some wells in Amara oil field. Table (6) illustrates the cost reduction related to cement.

Drilling Fluids Cost
The cost data related to drilling mud of X-1 is available and it's obtained from MOC. The calculations based on the price of ton of material. It should noticed that 1 bag of material is equal to 25 Kg.

Overall Cost Reduction
Based on the costs that have been taken into consideration, the total cost to be reduced from drilling one well is around 300,000 USD.  2. Damage of the 9 5/8˝, 47 ppf, P-110, BTC production casing probably occurred after the 2 nd stage of cementing and the casing damage was at the depth 32 m in the pipe body (not in the couplings).