Water block

What is a water block?

A water block is a form of formation damage in which water accumulates in the rock around a wellbore or hydraulic fracture and restricts the flow of oil or gas. Rock that is filled with more water than normal has less space through which hydrocarbons can move, and the water can be difficult to dislodge. The condition is also called water blocking, phase trapping or aqueous phase trapping.

How water blocks form

Water blocks form when water-based fluids enter the rock and are not fully produced back. Common sources are:

• Fracturing fluid that soaks into the rock next to the fractures during hydraulic fracturing.
• Drilling, completion and workover fluids that invade the rock near the wellbore.
• Water coning or influx from nearby water-bearing zones.

Once in the rock, water is held by capillary pressure. The smaller the pores and the higher the interfacial tension between the water and the hydrocarbons, the harder the water is to displace. Rock that prefers to be in contact with water, a water-wet wettability, holds it especially strongly in its fine pores.

Why tight rock is most vulnerable

Water blocks are a particular concern in tight gas sands and shale, where pores are tiny and capillary pressures are very high. In these rocks the pressure available to push water out of the pores may not be enough to overcome the capillary forces holding it, so the damage can persist. Low reservoir pressure makes the problem worse, because less pressure is available to clean up the rock.

Effects on production

A water block reduces the rock’s effective permeability to oil and gas where it matters most: at the fracture face and near the wellbore, where all production must pass. Symptoms include slow or incomplete cleanup after a treatment, lower-than-expected initial production, and low load recovery of injected fluids during flowback.

Preventing and removing water blocks

• Surfactants in fracturing and completion fluids lower interfacial tension and alter wettability, reducing the capillary forces that trap water so that more of it flows back.
• Alcohols and solvents, such as methanol, are used in some gas wells to lower interfacial tension and help remove trapped water.
• Minimizing fluid loss and using less water-sensitive fluid designs reduce how much water enters the rock.
• Time and drawdown. Some water blocks clear gradually as the well produces, but in tight rock that can take a long time or remain incomplete.

Water blocks and Locus biosurfactants

Locus describes its SUSTAIN frac surfactant as addressing capillary trapping and water blocks by reducing interfacial tension and capillary pressure, and positions its AssurEOR FLOW treatment for near-wellbore cleanup.

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