CASE STUDY APPLYING DYNAMIC UNDERBALANCE TECHNIQUE This case study has been taken from SPE/IADC 97363: “Dynamic Underbalance

Perforating System Increases Productivity & Reduces Cost in Kalimantan Gas Field: A Case Study, D. Minto, P. Falxa, D. Manalu, and M. Simatupang, Total E&P Indonesia, and L.A. Behrmann, SPE, A. Kusumadjaja, SPE, Schlumberger, presented at the SPE/IADC Middle East Drilling Technology Conference & Exhibition held in Dubai, U.A.E., 12-14 September 2005.”

The Tunu Field, which is the major gas field of East Kalimantan, is situated in the eastern limit of the Mahakam Delta (Figure 3.8). Here operators, Inpex and Total E&P Indonesia are working along with other fields such as Bekapai and Handil oil fields, Sisi-Nubi and South Mahakam Gas Condensate fields around the region. The Tunu Field is 75 km long and 15 km wide, consisting of sandstone formation between a depth of 2,300-4,500 m.

Production started in 1990, with 1.3 billion cubic feet per day (Bcf/Day) and 23,000 stock-tank barrels per day (STB/Day) of condensate in 2004.

Figure 3.8: Tunu Field of East Kalimantan in the Eastern Limit of Mahakam Delta (Source: D. Minto et al, 2005)

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Previously new gas wells in the Tunu Field were perforated using a static underbalance technique, in which a large static underbalance pressure based on reservoir and wellbore characteristics was set before the firing of the gun. This was causing much higher skin, formation damage, high cost and time consuming activities like acidization after perforation for cleaning purposes. Therefore a new approach, dynamic underbalance perforation based on the maximum underbalance pressure during the perforation process, was recommended. As discussed in the previous section it is the key factor in governing the perforation cleanup, high production rates, and consequently low skin effect. Thus, here we will discuss the increase in production and the low skin produced in two Tunu wells using the dynamic underbalance approach. Reservoir properties for the two Tunu wells, Tunu #1 and Tunu #2, can be found in Table 3.2.

Reservoir Pressure (Psia) 5,586 5,234

Initial Wellbore Pressure (Psia) 5,113 5,061

Perforation Interval Depth 3,489.0 – 3,492.5 3,454.1 – 3,458.1

Table 3.2: Reservoir Properties for the two Tuno wells

The initial wellbore pressure was found to be 5,113 Psia and 5,061 Psia for Tunu # 1 &

Tunu # 2 respectively. As a result the wellbores were in static underbalance conditions due to low wellbore pressure values than the average reservoir pressure. In order to determine optimum perforating performance many perforating systems were evaluated depending upon the reservoir and the two wellbore downhole data (Table 3.1). These systems consist of parameters like size of the gun, shot density, phasing and type of

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charges. The evaluation was done with the help of a perforating simulator. Perforation depth estimation was based on the field test data, obtained in offsetting wells and previous client studies relating to current dynamic underbalance perforating practices.

An optimal perforating system was selected as per API 19B specifications to achieve the dynamic underbalance perforation Table 3.3. After selecting the optimal gun perforating system the parameters were entered into patented dynamic underbalance software (different for different companies) especially designed to give design dynamic underbalance pressure, firing pressure and productivity along with cost evaluation of the process. Table 3.2 a summary of dynamic underbalance design pressure data.

Parameters Tunu # 1 Tunu # 2

Reservoir Pressure (Psia) 5,586 5,234

Detonation Pressure (Psia) 5,113 5,061

Dynamic Underbalance (Psia) 3,250 3,000

Guns Diameter (in.) 2-7/8 2-7/8

Shot Density (spf) 6 6

Phasing 60 60

Penetration Depth (in.) 25.3 25.3

Table 3.3: Dynamic Underbalance design pressure data

The gun was then conveyed down in the wellbore through wireline string. During detonation the transient underbalance pressure occurring during the first 200 millisecond of detonation couldn’t be accurately recorded. It was due to malfunctioning of the pressure gauges at that time on the field. Figure 3.9 shows the pressure profile of well Tunu#1 during dynamic underbalance perforation. A dynamic underbalance of around 2900 Psia is achieved which is near to the value given by the software (Table 3.3).

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Figure 3.9: Dynamic underbalance effect during the perforation process of well Tunu #1 (Source: D. Minto et al, 2005)

Within 30 minutes of detonation, wellbore fluid with debris was observed on the surface equipment. After connecting it to the production line, the production rates for both wells were observed and found to be 15 MMscf/D and 25 MMscf/D with wellhead flowing pressure (WHFP) of 435 Psi. Post perforation results can be found in table 3.3

Table 3.4 – Post Perforation Results for Tunu #1 and Tunu #2

These skins in Table 3.4, obtained by the evaluation of data from pressure build up test (PBU) after dynamic underbalance perforation, are compared with the skins from static underbalance perforation. According to this case study, skin values for only 4 wells out of 10 could be evaluated from PBU test. The average skin value of those 4 wells is -0.27 (Figure 3.10). Whereas the average skin value for the previously drilled 35 wells (2000-2004) using static underbalance technique was less than 5 (D. Minto, 2005).

Wells Reservoir Pr. (Psi)

Wellhead Flowing Pr.

(Psi)

Q (MMscf/D) Skin

Tunu 1 5,000 435 15 1.1

Tunu 2 4,598 435 25 0

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Figure 3.10: Comparison of Skin Values between Static Underbalance & Dynamic Underbalance (Source: D. Minto et al, 2005)

The Inflow performance of a well is the function not only of reservoir permeability, fluid viscosity, and the well drainage area, but also the formation damage linked with the mechanical skin. The inflow productivity of a well is the flow rate divided by the pressure drawdown (Mason et. al., 2005). Therefore the Inflow Performance Curve (IPR) in Figure 3.11 for well Tunu #2, shows the different production rates with varying skin values, illustrating that with skin values from 1-0 the production rates are almost 60%

greater than those with skin values from 1-5.

Figure 3.11: Impact of Skin Value to the IPR Curve for Well Tunu#2 (Source: D. Minto et al, 2005)

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According to this study regarding cost and expenditure, 10 Tunu wells perforated by the dynamic underbalance approach have resulted in a cost saving of around 40% as compared to the previous wells perforated using the static underbalance technique with more operational and time cost. This cost was due to the rental of the equipment for fluid loading/unloading in tubing in order to develop static underbalance conditions before perforation. This activity also requires more time delay and personnel.