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Seismicity has been interrupting production of DMLZ (Deep Mill Level Zone) block-cave mine. B-value analysis shows that seismic mechanism is caused by mining excavation. Excavation such as undercutting and mucking, triggers stress redistribution in rock mass. The seismic location plot with certain magnitude for recent six months indicates seismic is concentrated in the southern or active abutment consisting of diorite lithology. Subsequently, the lateral seismic evolution per a certain period also shows seismic concentrated in the southern abutment in the beginning and moved finally to the western abutment but seismic energy is concentrated in the southern abutment. Vertical seismic evolution shows seismogenic zone peak rises proportionally to production activity rate. A graph between seismicity and mining production shows seismic background rate for non-production activity and this rate can increase to 6-10 times because of production activities. Seismic events can trigger damage in rock mass called rockburst. Rockburst potency in rock mass is assessed by three parameters: strain energy density (SED), σc/σ1, and burst prone index (BPI). SED shows diorite and limestone are high rockburst rocks but skarn has the lowest rockburst potency. Ratio of σc/σ1 shows high rockburst potency occurs in three DMLZ rock masses. BPI in the tunnel vault is higher than the sidewall with the value is inversely proportional to cave front distance. Next, the back analysis of major rockburst cases indicates rockburst occurred in the active abutment that had not been undercut for a month or no undercutting activities. In short, objectives of this paper are to identify the cause of mining seismicity, divide abutment based on seismic and lithology, analyze changes seismic as well as energy concentration per a period laterally and vertically, observe relationship between mining production and seismicity, and analyze rock mass causes as well as tendency to rockbursting. Three advancement things of this research compared to the previous ones: it shows abutment division based on lithology and seismic, the relationship between seismicity to mining production, and solutions to minimize rockburst.
Sajjad, Farasdaq (PT Pertamina Hulu Energi Offshore Northwest Java) | Chandra, Steven (Institut Teknologi Bandung) | Naja, Savinatun (PT Pertamina Hulu Energi Offshore Northwest Java) | Suganda, Wingky (PT Pertamina Hulu Energi Offshore Northwest Java)
We present a simple analytical solution to diagnose gas production under compaction. This solution scales production profile of different wells and collapses them into a single general curve. The curve will later serve as the "learning" function for physic-based machine-learning prediction.
A rapid growing flood of big data in the oil and gas industry reveals a substantial opportunity to the better understanding of hydrocarbon reservoir. With machine learning, one can turn a numerous amount of data to predict future production and determine field economics. However, the quality of the prediction from machine learning is dependent on the learning function selected that most of the time does not concatenate any physical aspects of the problem. In this paper, we offer a better machine learning with a physics-based function to estimate future gas production under severe compaction.
We construct a physic-based master curve by solving the coupled Darcy-Biot equation for vertical gas well under reservoir compaction. We assume that the flow is radial and the porosity is transiently changing by the reduction in pore pressure due to gas production. Finally, we reduce the complexity of the coupled non-linear equation to two scaling optimization parameters: a mass scaling factor to scale the recovery factor and time scaling factor to scale the diffusion time.
We verify our model with a field case from KLX field, Indonesia. This gas field produces an enormous amount of gas with subsidence as the side effect. The subsidence was identified by knowing the change in platforms level. By collapsing the production profile of all existing wells into a single master curve, we capture the universal scaling parameters that represent the behavior of gas flow under reservoir compaction. Furthermore, we can substitute the resulted master curve as the learning function for to the machine-learning model to predict and diagnose other fields in the future that undergo the same phenomena.
We find that reservoir compaction leads to a higher recovery factor of gas for a long term. However, the high subsidence rate is not a favorable condition for the offshore field as the production facilities on the platform will submerge under sea level in a matter of years. Thus, the field owners must consider some subsidence mitigations such as injection and maintaining critical production rate.
Our novelty is to produce a general scaling to describe gas production under compaction, which is later useful for the development of our machine-learning process to simplify the prediction process, not involving extensive and expensive numerical simulation.
Muchibbus Sajjad, Farasdaq (PT Pertamina Hulu Energi Offshore North West Java) | Wirawan, Alvin (PT Pertamina Hulu Energi Offshore North West Java) | Naja, Savinatun (PT Pertamina Hulu Energi Offshore North West Java) | Suganda, Wingky (Institut Teknologi Bandung) | Pramana, Harris (Institut Teknologi Bandung) | Indro, Axel Perwira (Institut Teknologi Bandung) | Chandra, Steven (Institut Teknologi Bandung)
Offshore North West Java is a mature oil and gas field located in northern part of Java Island. Most of the wells are producing with gas lift system from the abundant source of gas in the field. Through forty years production life of this field, the conventional gas lift spacing design is found out to be not optimum. Many unloaders at the upper part of the completion aren't necessary during its production stage and usually will be changed into a dummy valve during gas lift valve redesign (GLVR) operation. These excessive number of valves may cause many problems, such as limited gas that can be delivered through the orifice, higher probability of valve and installation failure, and many more. These conditions will lead to un-optimum production rate.
A new innovative method of gas lift spacing design is proposed to solve the problem by optimizing the number of gas lift valves installed in the completion. In conventional gas lift spacing design, completion fluid level is often represented static at the surface using a static fluid model. In fact, completion fluid level tends to change over time due to fluid infiltration into the reservoir. By emphasizing this fluid infiltration into the reservoir, equalized method is created. This equalized method alters the spacing design starting depth from the surface into the depth which equalized condition between bottom hole and reservoir pressure is reached. By combining Darcy' law and hydrostatic pressure formula, a new equation is derived. It is able to forecast the time needed to reach the equalized depth and also the depth itself.
To verify the newly developed method, a case study of Well-X is presented. To enhance Well-X production, a gas lift system is required. Using a predetermined compressor pressure, the conventional gas lift spacing method yields a total of eight unloader valves. In contrast, the equalized method reduced the number of unloader valves required to a total of four. The example has proved that the equalized method is not only able to reduce the chance of failure in the installation, but it also results in a higher gas lift operating pressure, higher gas injection capacity, and in the end, 5.6% of higher oil production rates obtained compared to the conventional method.
The novelty of this paper is an optimized gas lift spacing design by using the equalized method. For further implementation, this method can be applied in most oil well cases with gas lift system, for a better economic profit.
Sajjad, Farasdaq Muchibus (PT Pertamina Hulu Energi Offshore North West Java) | Hadi Prasetyo, Abraham (PT Pertamina Hulu Energi Offshore North West Java) | Chandra, Steven (Institut Teknologi Bandung) | Wijaya, Budi Rivai (PT Pertamina Hulu Energi Offshore North West Java) | Naja, Savinatun (PT Pertamina Hulu Energi Offshore North West Java)
Abandonment and site restoration (ASR) is one of the responsibilities of Oil and Gas Company for the country where it operates. The ASR covers plug and abandon (P&A) of wells and dismantling of surface equipment in order to restore the environment as close as possible to its original state. Current practices in Indonesia's oil and gas business does not put P&A as one of the top priorities, compared to other engineering aspects in petroleum engineering. Therefore, P&A is sometimes regarded as a formality. A case study where long term oil and gas exploration and production with unique rock mechanics in Offshore North West Java (ONWJ) Area has caused subsidence and inherently leading to both production operation and well integrity issues. Several issues namely casing deformation and detachment from main casing strings have been observed and is likely to put harm to oil and gas production in ONWJ Area. Casing issues such as buckling and crumpling, as well as the presence of micro cracks will present complications in safely plugging and abandoning the affected wells. Recommendations based on current practices around the world as well as mitigation solutions done in ONWJ Area such as mechanical stress and/or strain release combined with well condition evaluation and monitoring is maximized as an input to properly design safe and cost efficient P&A strategy for complex, marginal, and offshore wells with integrity issues. This research is aimed to become a benchmark for future uses of P&A not only in ONWJ area but also in Indonesia.
Apranda, Yoseph Robby (Institut Teknologi Bandung) | Riadi, Ridha Santika (Pertamina Hulu Sanga-Sanga) | Nugraha, Teguh (Pertamina Hulu Sanga-Sanga) | Permana, Robhy Cahya (Pertamina Hulu Sanga-Sanga) | Putranto, Asnanto Multa (Pertamina Hulu Sanga-Sanga) | Noerad, Dardji (Institut Teknologi Bandung)
The Sanga-Sanga working area consists of brown fields that have been produced for nearly 50 years. The production is declining rapidly from anticlinal trap fields. Finding new resources is a must in order to extend the production life. Therefore, the exploration requires getting deeper targeting reservoirs associated with hard overpressure zones.
The methodology used to identify and recognize the potency of hard overpressure zone is the integration of geology, geophysics, and geochemistry data. Normal compaction trend and pore pressure analyses were performed to determine top of overpressure zones. The geochemical data from biomarker provide calculated vitrinite reflectance (Rc) to complement vitrinite reflectance (Ro) when evaluating relationship between hydrocarbon origin and overpressure generation. Finally, the seismic integrates all the data in structural reconstruction framework.
Pore pressure analysis showed 9 wells have overpressure zones. The overpressure occurrence can be grouped based on anticlinal lineaments. The Badak-Nilam lineament is characterized by a sharp change in overpressure to hard overpressure zone, distal facies, relatively normal deposition and showing higher pressure gradient. The Semberah and Lampake-Mutiara lineament are characterized by long transition zone, proximal facies, strongly uplifted and folded, and showing lower pressure gradient. Ro and Rc data showed that there are two periods of hydrocarbon charging into the reservoir, prior to and after the hard overpressure zone occurred. Vertical effective stress, dynamic mechanism, and the timing hydrocarbon generation-migration and overpressure generation hole significant role to accumulation.
The evaluation of hard overpressure zone play requires the integration of geology, geophysics and geochemistry to get the understanding of the timing sequence between hydrocarbon generation-migration and overpressure generation. Calculated vitrinite reflectance data from biomarker gave an advantage to this timing relationship that could lead to the exploration of the hidden resources in the hard overpressure zone becomes more feasible.
Tunu Field is a giant gas field located in the Mahakam Delta, East Kalimantan, Indonesia. Tunu Main Zone is the biggest gas contributor of this field, which is dominated by delta front mouth bar sands with some delta plain distributary channel sands. One of the uncertainties in Initial Gas In-Place (IGIP) calculation is coming from water salinity used in the formation evaluation. Observation of the water salinity values over Tunu Main Zone shows a distinctive decreasing pattern (the deeper the burial depth, the smaller the water salinity value is). This condition attracts attention to get a better understanding on the genesis of water salinity. Any alteration related to the different genesis will have sensitive impact on the salinity values.
Integrated analysis of depositional environment facies, water chemistry and clay mineralogy is required to characterize the genesis. Therefore, the following available and valuable data of 75 Tunu wells have been used in this study, i.e.: water chemistry (water salinity, isotopes and anion-cation content), E-logs and core data.
The results show that there is a decrease in water salinity trend by burial depth in the Tunu Main Zone. It is caused by the contribution of water expelled from shale (because of compaction and transformation of Feldspar into Kaolinite) and entering the Tunu Main Zone reservoirs. This process is interpreted as a reverse osmosis, i.e. water flows through a semi-permeable membrane from higher salinity region in the eastern side of the field to lower salinity region in the western side. In the study area, the shale acts as the semi-permeable membrane. During the water expulsion, the salts that dissolved in the shale water cannot penetrate the shale because its molecular size is relatively bigger, thus only water molecules can be expelled from the shale. As the consequence, the expelled water has very low salinity, then mixed with reservoir water (originally more saline). This circumstance causes decreasing of water salinity in the reservoirs in order to balance the reservoir hydrodynamics.
Tunnel collapse in weak rock masses occurs when the support pressure given by the installed support system is not adequate to sustain the weight of broken rock resulting from the excavation. Further, installation of support system will encounter great difficulty when the maximum allowable limit of strain before setting up the support is exceeded. This paper presents the results of numerical simulations in computer code FLAC to integrate the use of ground reaction curve and equilibrium strain approach for designing tunnel support system in a non-circular excavation. The equilibrium strain is used as one of the design tools because the final radial deformation of a supported tunnel occurs at the equilibrium between the support and the deforming ground, not at the time of support installation. The approach is demonstrated on a large cross-section road tunnel that is excavated through weak rock masses with varying qualities. Results from the FLAC model show that when the time to install support system is correctly estimated at the equilibrium strain εeq of 1%, the amount of vertical deformation and the extent of the plastic zone around the tunnel decrease significantly. This correct estimation is made possible with the help from the ground reaction curve (GRC). Moreover, as shown by the support capacity diagrams, the induced bending moment, axial and shear loads in the tunnel lining are well inside the strength envelopes of the support system with factor of safety > 1.5. Results from this research indicate that the integrated use of GRC and equilibrium strain approach can be used as a tool to achieve a reliable tunnel support design.
Designing a tunnel support system in weak rock masses must cope with the estimation of support pressure that is needed to stabilize the tunnel face and the newly-opened room behind the face. In this situation, collapse of the opening occurs when the support pressure given by the installed support system is not adequate to sustain the weight of broken rock resulting from the excavation. In particular, installation of support system will encounter great difficulty when the maximum allowable limit of strain before setting up the support is exceeded.
The so-called convergence-confinement method (CCM) has been a standard practice for evaluating the displacement behavior of a tunnel and to determine the required support pressure to control the convergence of the tunnel wall (Carranza-Torres and Fairhurst, 2000; Vlachopoulos and Diederichs, 2009; Prassetyo, 2017; Prassetyo and Gutierrez, 2018). The CCM consists of the ground reaction curve (GRC), the support reaction curve (SRC), and the longitudinal displacement curve (LDP). The GRC represents the relationship between the increasing radial displacement of the tunnel wall ur and the decreasing internal support pressure pi. The SRC represents the relationship between the increasing support pressure ps and the increasing radial displacement of the support us. The last component, the LDP, represents the radial displacement occurring along the longitudinal axis of the unsupported tunnel.
Biot’s theory of poroelasticity has gained new prominence in rock mechanics to understand the hydro-mechanical (H-M) response of fluid flow and deformation in tunneling in deep saturated ground. Numerically, explicit coupling technique has been widely used for simulating this coupled interaction. However, the technique is conditionally stable and requires small time steps, making it inefficient for simulating large-scale H-M problems. To improve the efficiency, the unconditionally stable alternating direction explicit (ADE) scheme could be used to solve the flow problem. The standard ADE scheme, however, is only moderately accurate and is restricted to uniform grids and plane strain problems. Thus, it is impractical for large-scale domains and inapplicable for axisymmetric problems. This paper aims to remove these drawbacks by developing a novel high-order ADE scheme capable of solving the flow problem in an axisymmetric non-uniform grid. The new scheme is derived by performing a fourth-order finite difference approximation for the spatial derivatives of the axisymmetric fluid-diffusion equation in a non-uniform grid configuration. The implicit Crank-Nicolson technique is then applied to the resulting approximation, and the subsequent equation is split into two alternating direction sweeps, giving rise to a new high-order axisymmetric ADE scheme. The pore pressure solutions from the new scheme are then sequentially coupled with an existing geomechanical simulator in the computer code Fast Lagrangian Analysis of Continua (FLAC). This coupling procedure is called the sequentially-explicit coupling technique based on the fourth-order axisymmetric ADE scheme or SEA-4-AXI. When applied for simulating an advancing tunnel in deep saturated ground, SEA-4-AXI reduces computer runtime up to 42% that of FLAC’s basic scheme without numerical instability while also producing high numerical accuracy with average differences of 0.6-1.8% for pore pressure and displacement.
Biot’s theory of poroelasticity (Biot, 1941) has gained new prominence in rock mechanics to understand the coupled response of fluid flow and deformation in deformable porous media (i.e., soils and rocks). Consolidation and subsidence induced by fluid extraction from underground formations are the most common examples of this coupled hydro-mechanical (H-M) interaction. In these examples, the transient fluid flow affects deformation in the ground and vice versa (Wang, 2000; Gutierrez and Lewis, 2002; Neuzil, 2003). Thus, consideration of this H-M interaction is essential for the safe design of underground structures such as deep tunnel in saturated ground.
Barton’s joint model is the most realistic model for predicting the nonlinear shear behavior of rock joints. This capability comes from the inclusion of a joint surface roughness parameter called the joint roughness coefficient (JRC) that is mobilized under shearing. Recently, a linearized implementation of Barton’s model has been done to obtain the mobilized equivalent Mohr-Coulomb (M-C) parameters that account for generation and reduction of JRC as a function of shear displacement ∆u. These equivalent parameters will allow the linear M-C model to capture nonlinearity in the shear behavior of rock joints. In the linearized Barton’s model, the pre- and post-peak joint shear stiffness also contains mobilized JRC that is expressed as hyperbolic and logarithmic functions of ∆u, respectively. This paper further explores the capability of the linearized Barton model to predict the shear behavior of rock joints. The model is verified against results from the experimental and numerical direct shear test on joint planes from various rock types. The verification shows that the linearized Barton’s model can capture the nonlinearity in the shear stress-displacement and in the dilation-induced shear displacement behaviors of rock joints under variations normal stress and JRC values. In the future, the linearized Barton’s model has the potential to be applied in computer codes for fractured rock modeling. By implementing this model, neither the simplicity of the linear M-C model nor the advanced capability of the nonlinear Barton’s model is lost.
In a fractured rock mass, the shear behavior of rock joints is particularly important because it dominantly controls the deformability, strength, and hence the stability of the rock mass. Block sliding from a slope or block falling in an underground excavation are examples of joint shear behavior that is not only controlled by the shear strength of the particular joint but also by its dilation (Goodman, 1976; Barton, 1982). The dilation is caused by the mobilization of joint surface roughness, causing a nonlinearity in the shear behavior of rock joints in the form of strain hardening and strain softening under shearing.
Drill and blast is the excavation method adopted to remove overburden material at the open pit coal mine of PT Buma Job Site Lati. Recently, the company applied deep hole drilling for blasting with double rods to reach the depth of 10-18 m. The main explosive was an emulsion explosive with target explosive consumption of 0.23 kg/m3. The blast hole was not fully charged but vertically decoupled using air decks at the middle and bottom of each blasting hole. Blasted rocks were then measured by digital photograph, and the size of P80 was found to range from 200 to 800 mm. The evaluation results indicate there is close relationship between the explosive consumption and the fragment size as well as the digging time. The air deck technique adopted in this study has been giving good results in terms of fragmentation size and explosive consumption.
In mining operations, drilling and blasting with a deep hole is a preferred method to reduce lost production time caused by the delays in blasting, to increase blasting inventory, and to minimize the number of drill pads so the drilling deviation can be minimized as well. Recently, PT BUMA Job Site Lati (called BUMA from now on) applied drilling for blasting operations with double rods to reach the depth of 10-18 m. Consequently, explosive consumption was high in the lower part of the blast hole, then an air deck was used to distribute the explosive along the blast hole. Another benefit of the air deck was that it cut the waiting time of the on-site sensitized explosive expansion to accommodate gassing.
In this study, the performance of blasting operation using the air deck is evaluated in terms of fragmentation. This study aims to check the effectiveness of using an air deck with the productivity target at the particular mine and to develop a blasting-fragmentation model that can be used to predict the size of the fragmented rock.
2. Literature Review
The air deck method is well-known in blasting operations to improve the quality of the blasting results. In the early 1940s, Russian scientists first came up with the idea of using an air gap between explosive columns. This method reduced explosive consumption in blasting activity. Melnikov et al. (1979) mentioned that an air deck can act as an energy accumulator. Marchenko (1982) found that pressure in an air deck would expand micro fractures that were previously generated by the main shock wave during blasting. Pompanna and Chikkareddy (1993) concluded that the presence of an air gap in the blast hole can reduce ground vibration and back break at the Kudrremukh iron mines. Jhanwar et al. (1996) revealed that the mechanism of air deck can reduce 25-30% of explosive consumption. Chiapppeta (2004) conducted experiments in the field and found that the air deck technique could remove the sub-drill which in turn reduced the explosive consumption by 16-25%, decreased vibration due to blasting by 33%, and improved fragmentation by 25%.
Utilization of an air deck will increase the fracture network due to the secondary shock waves formed as the result of wave reflection in the air gap. The fracture degree increases as a result of secondary shock waves as the duration of the shock wave effect on the rock mass around the blast hole becomes longer. The pressure reflections from the upper and lower explosive columns will collide in the middle of the air deck and is expected to interact with the surrounding rock mass to form additional radial fractures (Moxon et al., 1993; Zhang, 2016; see Fig.1). Air deck methods have been used in some open pit mines to reduce the consumption of explosive and to improve fragmentation (Chiapetta, 2004).