Counter Pressure Trapping Mechanism in the Offshore Mahakam Gas Fields

Counter pressure trapping is an important mechanism for hydrocarbon accumulation in the Mahakam Block, particularly in the P and S Field. This trapping mechanism occurs due to hydrodynamic forces caused by the expulsion of water from shale formations. As hydrocarbons migrate through the reservoir rock, they encounter shale barriers that act as seals, preventing further upward migration. In our field, counter pressure trapping occurs in tilted hydrocarbon-water contacts. As the hydrocarbons accumulate in the reservoir, they create a pressure gradient that causes the hydrocarbon-water contact to tilt, with the hydrocarbons accumulating in a higher position in the reservoir. This creates a counter pressure that balances the pressure of the hydrocarbons, effectively trapping them in place.

This figure present fundamental concepts that are crucial in understanding hydrodynamics. The comparison of fluid dynamics in hydrostatic and hydrodynamic phases is illustrated in the picture.

Hydrostatic conditions refer to a state where fluids within a geological formation or reservoir are at rest or in equilibrium with the surrounding pressure. This means that the pressure of the fluid is uniform and does not change over time. In such conditions, the movement of fluids is typically driven by gravity, and the flow of fluid is relatively slow and laminar.

Hydrodynamic conditions, on the other hand, refer to a state where fluids are in motion due to the influence of external forces such as the movement of water or pressure gradients within the reservoir. In hydrodynamic conditions, the pressure of the fluid is not uniform and can vary over time and space, leading to the movement of fluids in complex patterns. The flow of fluid in hydrodynamic conditions is typically faster and more turbulent than in hydrostatic conditions.

Understanding the differences between hydrostatic and hydrodynamic conditions is important in geology because it can help geologists to understand the movement and accumulation of fluids such as oil, gas, and groundwater. By analyzing the pressure and flow patterns within geological formations, geologists can identify potential areas of fluid accumulation and develop strategies for the exploration and production of these resources.

The Hubbert concept is a well-known concept in petroleum geology, introduced by M. King Hubbert in his 1953 paper "Entrapment of Petroleum under Hydrodynamic Conditions". Hubbert proposed that hydrocarbons can accumulate in tilted traps due to the movement of water and hydrocarbons through permeable rock formations.

The basic idea behind the Hubbert concept is that hydrocarbons can be trapped in a tilted or faulted reservoir due to the hydrodynamic forces generated by the movement of water through the reservoir. As water flows through the reservoir, it creates a pressure gradient that can cause hydrocarbons to migrate towards the low-pressure areas. If the reservoir is tilted or faulted, the hydrocarbons can accumulate in a trap where the tilted hydrocarbon-water contact intersects with a sealing fault or impermeable layer.

By mapping the tilted hydrocarbon-water contact, geologists can identify areas where hydrodynamic traps may exist and where hydrocarbon resources may be present. The Hubbert concept has been widely used in the petroleum industry to identify potential hydrocarbon reservoirs and to develop exploration and production strategies.

Sedimentary basins are geological structures that form due to the accumulation of sediments over time. These basins can contain various types of fluids such as water, oil, and gas, which can be trapped within the rocks and sediments. Gravity-driven hydrodynamic trapping (left figure) occurs when fluids migrate through the sedimentary rocks due to pressure differences and are trapped in a low-permeability layer or a structural trap such as a fault or fold. This process occurs when a fluid, such as water, oil, or gas, is under pressure and migrates through the permeable rocks until it encounters a barrier, such as a low-permeability layer or a structural trap. When the fluid encounters this barrier, it is prevented from further movement, leading to its accumulation in the trap.

Counter-pressure trapping due to compaction (right figure) occurs when sedimentary rocks are compacted due to the weight of overlying sediments, leading to an increase in pore pressure. In this scenario, the fluid is trapped due to the balance between the weight of overlying sediments and the pore pressure of the fluid. The pore pressure of the fluid acts as a counter-pressure to the weight of the overlying sediments, preventing the fluid from being expelled from the sedimentary basin.

During the process of sedimentation, sediments are deposited on top of each other, leading to the compaction of the underlying layers. As the sediments are compacted, the pore space between the grains is reduced, causing an increase in pore pressure. This increase in pore pressure can act as a counter-pressure, preventing hydrocarbons from escaping the reservoir. In this case, pressure differences can cause the movement of hydrocarbons to downdip areas, impacting the accumulation of hydrocarbons in these areas.

The tilted hydrocarbon water contact in the Mahakam Block is thought to be the result of counter pressure trapping, which is driven by compaction mechanisms. In certain offshore gas fields, tilted hydrocarbon accumulations can be observed where gas accumulates along the field, creating a mismatch between structural closure and hydrocarbon accumulation.

How can we identify evidence of counter pressure trapping in our field? Here are some findings related to this matter.

The first evidence is the variation of hydrocarbon-water contact in the reservoir. In the left illustration, we can see that the evidence of this mechanism can be observed from the presence of a gas gradient plot of hydrocarbon lines accompanied by several lines of water gradient in one pressure gradient plot in the same layer.

Lateral reservoir drainage refers to the process of hydrocarbons moving laterally (horizontally) through the reservoir rock, as opposed to vertically. The shoulder effect can impact lateral reservoir drainage by creating barriers to the movement of hydrocarbons. In a reservoir with good sandstone lateral connections, hydrocarbons can move laterally through the rock, aided by the pressure gradient created by the movement of fluids through the reservoir. However, if there is a shoulder effect present, the pressure gradient acting as a barrier to hydrocarbon movement will be strongest at the edges of the accumulation. This can create a situation where hydrocarbons are trapped in the central portion of the accumulation, unable to move laterally beyond the shoulders.?

The phenomenon of tilted-hydrocarbon water contacts has garnered significant interest due to its association with the overpressure mechanism, as evidenced by several studies. Ramdan et al. (2018) have identified the tilted-hydrocarbon water contacts in Lower Kutai Basin as a hydrodynamic trap resulting from lateral reservoir drainage. Our field's pressure plot data (Fig.7) has provided clear evidence of counter pressure trapping, manifested by the shoulder effect observed in all panels (Panel 1 and 2) of the S field. The L13, L16, and FS9.55b layers, which possess strong sandstone lateral connections, display the shoulder effect at regular intervals, further supporting the occurrence of counter pressure trapping in all panels of the S field.

The third piece of evidence is the lateral variation of the water pseudopotential, also known as water head. The water pseudopotential is a calculation of the water potential at a particular point or depth, and a water level at the surface, which has a water head potential of 0. This parameter provides insight into the level of overpressure, with higher water head values indicating higher overpressure levels.

The water pseudopotential is calculated to estimate the overpressure level of each well at a specific point, but only for water, as buoyancy effects are avoided when considering hydrocarbons. The figure on the right depicts the water head calculations for layer FS9.05 in the main panel of S field, obtained from several wells TL1, S1, S2, S6, and S5. The measurements reveal that TL1 has the highest pseudopotential water head, with decreasing values towards S5.

In summary, this article has provided a thorough discussion on the counter pressure trapping mechanism in Mahakam Block, along with the various evidence supporting its existence. The findings of this study provide new insights into our field development strategy in the near future.