Causes, Effects, and Control of Seepage Loss

Seepage losses are caused in highly permeable rocks. It is a continuous loss of drilling fluid into the formation through the pore throats. In general, a loss rate between 10 bbls/hour to 30 bbls/hour is considered a seepage loss. There could be other reasons also for encountering this small rate of mud loss. One reason is the retained drilling fluid on drilled solids. Since these drilled solids are removed from the system on the surface, the retained fluid is also removed. The effect is more pronounced if the drilling rate is high and the shale sakers are not efficiently removing the fluid on cuttings. In this case, the lost volume could be high enough to raise a suspicion of seepage losses. Another reason could be the hole enlargement in unconsolidated formations. This could create an impression that the hole is taking more fluid than the calculated volume and can be mistaken as a seepage loss. The situation should be evaluated to establish the real reason. Once the reason for loss is identified, proper remedial action can be taken.

A risk assessment and cost-benefit analysis should be carried out to evaluate if the cost of the seepage control measure outweighs the potential risks in the target zone. Many zones can be drilled with minimal or no seepage loss control, whereas others can pose severe risks. Depending on the stage of drilling operations, severity of losses, and type of drilling fluid, a decision should be made to initiate seepage loss control or ignore it. For example, if the hole is being drilled with inexpensive water-based mud, the loss rate is minimal and the hole section is reaching the casing point, the operator might decide not to take any action to control losses. Usually, the cost of mitigating seepage loss is minimal and hence is recommended even with moderate risks of it causing downhole troubles.

High seepage losses could have several adverse impacts as discussed below.

1.    Cost: The rate of seepage loss is usually less, but over time, a considerable amount of mud chemicals could be lost to the formation. If Oil Base Mud (OBM) or Synthetic Oil Base Mud (SBM) is used for drilling any sensitive zones, the overall cost of seepage losses could be significant.

2.    Formation Damage: In a reservoir formation, seepage losses could cause drilling fluid to invade pore spaces of reservoir rock, resulting in near-wellbore damage that would adversely affect the well productivity. It can also reduce the ability to evaluate logs.

3.    Stuck Pipe: The ideal mud design aims to deposit an impermeable and thin filter cake on the wellbore walls. High seepage loss tends to deposit a permeable and thick filter cake which causes downhole issues. Poor quality of filter cake could result in higher drag and tight hole conditions. It requires wiper trips and multiple circulations to condition the mud, which increases the overall cost of the well. If the filter cake is thick, it could cause the drill string to get stuck either differentially or mechanically, resulting in higher recovery time and well cost.

4.    Hole Enlargement: High seepage loss can cause hole enlargement in unconsolidated sand formations. Due to the pressure penetration, the differential pressure across the sand grains is lost causing the matrix on the borehole walls to collapse and enlarge the hole.

Controlling Seepage Losses:

Control of seepage losses is achieved through a combination of ‘Blocking’ material and ‘Filtration Control’ material. Only one of them on their own cannot provide effective control of the flow of fluid into the pore throats and hence both are needed in the mud system.

Blocking Material:  Seepage loss occurs when the pore throats of the formation are large enough to allow the flow of drilling fluid. Hence the primary requirement to stem the loss is to reduce this opening. Once the opening is sufficiently reduced, it can be plugged in easily. Blocking materials are bigger particles that tend to block the pore throats and accomplish this first step in the seepage loss control process. Drilled solids and barytes are the primary blocking material in the mud system which are lodged into the pore throats under the differential pressure and partially block the opening, reducing the rate of flow of drilling fluid into the formation. Blocking materials need to be of a certain size to create effective blockage. Once the bigger particles block the pore throats, finer blocking particles continue to build up until the gap is reduced to 1 – 5 microns.

Filtration Control Material: Even after blocking the pore throats with drilled solids and barytes, the filtration loss continues. Materials like ‘Bentonite’ and ‘Polymers’ are used as filtration control material in the Water Base Mud (WBM) and provide effective sealing of smaller gaps, completely stopping the seepage loss. Since the filtration control material is deposited on top of the layer of blocking material, it will be effective in providing a good seal only if the blocking materials provide effective blockage at the face of the formation and sufficiently reduce the gap. Hence a combination of blocking and filtration control material is required to achieve an effective control of seepage losses.

Filtration loss control works differently in the Non-Aqueous Fluids (NAF). NAFs have water particles in the dispersed phase. Since these dispersed water particles have high surface tension, they don’t easily pass through the smaller opening left by the blocking material and help achieve filtration loss control.

Engineered Particle Size:

This approach seeks to analyze, calculate, and mix the required sizes of particles in the mud system that can effectively plug the pore throats on the face of the wellbore. If the blocking particles are too small, they will pass through the pore throat opening and if they are too large, they will bounce off the wall and be circulated out of the well instead of getting lodged into the pore throats. There are several theories on the required particle size for blocking pore throat. A common field approximation is that particles can block the pore throats 2.5 to 3 times bigger than their size as depicted below.

This is also known as the “one-third blocking rule”. As the particles try to lodge into the pore throats under differential pressure, they create a bridge, reducing the gap. The flow of fluid through the pore throat continues but is reduced due to a reduction in the gap. Smaller particles accumulate to bridge and reduce these gaps further. Most companies recommend an ‘Optimized Particle Size Distribution’ approach by mixing different micron particles based on the above estimation.

The actual measurements of pore throats are hardly ever available. A complex set of air permeability analyses and mercury injection tests are needed on core plugs to acquire accurate measurements. Since neither all particles nor all pore throats are of the same size in reality, an estimation of pore throat diameter in microns can be used for selecting blocking particle size. Formation permeability information is readily available and can be obtained from either the Reservoir Engineer or the Geologist.

Although detailed equations and charts for accurate calculations of pore throat diameter for given permeability and porosity values are available, a workable estimate can be made using the permeability of the formation using the following rule of thumb:

Pore-Throat Diameter (microns) = Square Root of Permeability in milli-Darcy

For example:

If the permeability of a formation is 1.5 Darcy = 1,500 milli-Darcy

The Pore-Throat diameter ~ 38 microns

Hence the approximate size of the required blocking particle = 38/3 ~ 13 microns

Since barite is used as a weighting agent in most mud systems, it is easier and relatively less expensive to control seepage losses in high mud weight rather than while drilling with lower mud weights. The particle size distribution of barite plays the role of blocking material. API Barite has a particle size distribution between 0.5 – 100 microns with D50 about 15 – 20 microns. D50 is the particle diameter at which 50% of the particles are smaller and 50% are larger.

In a low-mud weight scenario, the fluid system will have to be supplemented with different particle sizes to be able to achieve effective blocking of pore throats. A drill and ‘seal pill’ with additional blocking material in the base fluid is a better strategy for low-weight mud systems.

In low permeability sands, a combination of very fine CaCO3 (5-10 microns) and a small amount of Asphaltene help form an effective base cake. If the permeability is in multi-darcy, a coarser blocking material is needed. CaCO3 serves as a good bridging material for reservoir formation as it is acid soluble and can be removed through acid wash while completing the well to reduce the ‘Skin’ effect.