Fatigue Failure Analysis of Bottom Hole Assembly during the Drilling of H-E1 and H-Q1 Wells in the Hululais Geothermal Field (Analisis Fatigue Failure Pada Bottom Hole Assembly Dalam Pemboran

In drilling operations it is intended to be able to conduct drilling quickly without causing damage to the drilling equipment and be able to produce good quality wells. Failure on the drilling pipe or bottom hole assembly (BHA) ranks results from loading that exceeds the maximum strength limit of the material. In this study, the calculation of the load on the downhole motor, where in the depth of the drill 512 meters failure occurs in the form of broken downhole motor. Based on the analysis conducted, at a depth of 494 meters the value of the hookload has increased quite high at 131.8 klb, while the maximum allowable hookload value is 121 klb, this indicates an overpull of 10.8 klb and based on the calculation of the drag value, a drag is obtained 38.8 klb while the maximum drag value is 28 klb, the difference between the actual drag value and the maximum allowable drag value is 10.8 klb, so it can be concluded that the failure that occurs in BHA # 10 is due to a drag that exceeds the maximum limit of 10.8 klb.


I. INTRODUCTION
The increase of the amount of world energy demand has an impact on the availability of oil and gas energy reserves. Therefore, in order to be able to support the energy needs new and / or renewable and environmentally friendly energy sources are needed. Technological developments and advances have had a positive impact on the discovery of alternative energy, namely the discovery of geothermal energy. Along with the development of energy utilization in the world, geothermal energy is considered to have many advantages, such as: renewable, environmentally friendly and relatively low cost when used by the community because the output generated from this energy is electrical energy, but not the drilling operation itself. Drilling operations in geothermal wells require substantial costs, so an effective and efficient technology is required to achieve the most economical geothermal drilling costs. The main challenges of geothermal well drilling are high pressure and temperature or better known as high pressure and high temperature (HPHT), then the type and character of rocks, where the majority of rocks in geothermal fields are igneous and metamorphic rocks, and hot reservoir gas earth that is corrosive. Therefore drilling equipment that is capable / resistant to these conditions is needed.
A drilling operation is designed to complete a drilling project quickly without causing damage to drilling equipment and is able to produce a good quality well. Damage / failure in the drill pipe and eISSN: 2614-0268 71 pISSN: 2615-3653 Bottom Hole Assembly (BHA) is caused by the imposition of excessive stress / stress on the drill pipe circuit. Drill pipe failure is often occurred in drilling of ultra-deep well at complex geological conditions [1]. Statistics indicate that about 50% of drillstring failures resulted from fatigue. Most drillstring failures occur on drillpipe and drill collar connectors or upset transition areas [2,3]. Fatigue is a gradually ongoing process, which brings along permanent structural changes on a localized level. Fatigue happens due to fluctuating or repeated strain at stresses having their maximum values well below the material yield strength used in designing the static limits of the drillstring [4]. Knowledge of the fatigue characteristics is necessary in order to achieve the analysis of dowhole tools under dynamicloads and reliable application of equipment without failure [5]. In this research, we will focus on calculating the load on the BHA which results in fatigue failure, where the application of the correct load / parameter will affect the service life and reduce the costs and drilling operation time.

II. METHOD 2.1 Data
The data needed in this research is in the form of actual data of drilling parameters, bottom hole assembly / BHA data, drilling plan data, and literature study. The data used in this study is obtained from PT Pertamina Geothermal Energi. This research was conducted on two wells located in Bengkulu Province. Drilling parameter data and BHA well data used are H-E1 and H-Q1 wells. In drilling a well, of course there are very many drilling parameter data and there are also a number of BHA that are used, in this study, in the H-E1 well the researchers only used data based on the initial depth of the well to the depth where the failure occurred. As for the H-Q1 well, the data used is adjusted to the data used in the H-E1 well. This is done to compare failure data with normal data (without failure).

Procedure
The initial stage in analyzing fatigue failure at BHA is to conduct a reference study that aims to find out the theories that cause fatigue. The next step is to collect the actual data of drilling parameters used in the field related to the failure that occurred. Then do the parameter calculations based on the existing fatigue failure theory. After the calculation results are obtained, the next step is to compare the calculation values with the actual data and reference study data. After that, the optimum parameter values and results are obtained.
The data processing in this study can be seen in the flow chart Figure 1. This research begins by collecting data, in the form of drilling data (hookload, WOB, torque, and messured depth), BHA data, and mud motor specification data. Calculating theoretical load values, including: maximum overpull, maximum Weight on bit (WOB), buckling load, bending stress, tensile stress, torque, and drag load. If the actual hookload value exceeds the allowable hookload value (motor specifications) and / or the actual torque load value is greater than the permissible torque value, failure will occur in the form of broken downhole motor or mud motor. If the actual load of the hookload and torque does not exceed the maximum allowable load, the BHA circuit is safe. The estimated value obtained is the maximum planning value that can be achieved when drilling takes place.

Calculation
Hookload parameter is used to determine the maximum tensile stress when tight holes or stuck pipes occur [6]. Hookload is calculated using the following equation, where Hk = hookload, lb W string = string weight in air, lb BF = buoyancy factor L = length of string, ft In stuck pipe conditions, the operator has to know how much additional tension, or pull can be applied to the string before exceeding the strength of the drill pipe. This parameter is known as overpull, since it is the pull force over the weight of the string. Therefore, the maximum overpull is defined as where Y = yield strength, lb Maximum weight on bit (WOB) is the maximum load that can be applied to the bit before failure happen. WOB can calculated using the following equation where WOB = weight on bit, lb SF = safety factor W BHA = BHA Weight in air, lb Buckling load is a force that works vertically on a drill string. This buckling load causes the pipe to bend due to pressure from above, in this case the pressure is to produce WOB [6], as in Figure 2. Dawson and ley, 1984 [7] introduced buckling load (FC) calculation in a rotated state. The equation is as follows, Drag load occurs due to the addition of the inclination of the wellbore that makes the drillstring to lean on the wall of the wellbore. When tripping in / out of the string, the friction is generated between the drill pipe and the wall. It causes the effect of compressive drag and tension drag on the drillpipe. The following equation can be used to estimate the drag load, where D = drag load, lbf Wm = buoyant weight = string weght (lb) x BF (lb/ft) L = string length, ft f = friction factor = 0.33 Table 1 shows the results of the calculation of the BHA load in the H-E1 well. Based on calculations that have been done, the hookload load of BHA # 2, BHA # 4, BHA # 5, BHA # 7, BHA # 8, and BHA # 9 are still within the allowable load eISSN: 2614-0268 73 pISSN: 2615-3653 limit. While for BHA # 10 at 494 m depth, the calculated load value of 93 klb, actual value of 131.8 klb, and the maximum limit value is 121 klb. This means that in BHA # 10 there was an overpull of 10.8 klb. This can be seen also in the actual drag load that occurs in the downhole motor that is 38.8 klb, where the maximum value of drag is 28 klb so this difference shows that the downhole motor is overloaded 10.8 klb from the maximum allowable drag load. Table 2 shows the results of the calculation of the BHA load in the H-Q1 well. Based on the calculations that have been made on the BHA used at H-Q1 well, we get hookload, WOB, drag, torque, buckling load, tensile stress, and bending stress that do not exceed the maximum allowable load. Accordingly, the drill pipe equipment used in the well does not undergo fatigue or even failure.

IV. CONCLUSIONS
From the results of the analysis from 4 wells to improve performance by strategy from KPI target as the objective to achieve performance.