The sand production rate in three outcrop sandstones under various saturation and flow fluids is investigated to reveal the potential contribution of three different mechanisms on sand production rate and initiation. The mechanisms that are investigated are strength water weakening, capillary cohesion in the failed zone and the pore pressure gradient front during water breakthrough. The sand production experiments are designed such that the relevant importance of each mechanism can be identified. Thus for each sandstone four sand production test types are performed and compared. Two of the test types are with one-phase flow and two are with two-phase flow including water breakthrough tests. The three tested sandstones are considered analogues of hydrocarbon reservoirs and represent all three borehole failure classes generalizing thus the applicability of the results which show that all mechanisms are important and have to be considered in the development of relevant sand production quantification models.

In hydrocarbon producing wellbores in sandstone reservoirs, sand production may occur due to the failure and subsequent erosion and transport of the rock around the wellbore. During production, the water below the hydrocarbons breaks through in the producing wells. Water breakthrough results from two reservoir-drive mechanisms: (i) Water drive where oil is driven through the reservoir by an active aquifer and where as the reservoir depletes, the water moves in from the aquifer below and displaces the oil, and (ii) Water injection increases the reservoir recovery factor and maintains the production rate for a longer period of time. This water breakthrough leads to increased sand production mainly in already sand producing wellbores. As more and more reservoirs enter their mature state, the risk of water breakthrough increases and the necessity to understand and model the effects of water breakthrough on the formation becomes more important. Three water-driven mechanisms of increased sand production are investigated experimentally and theoretically. The experiments pertain to uniaxial/triaxial compression tests and to Hollow Cylinder (HC) sand production tests with fluid flow. The investigated mechanisms were:

(i) Water-sensitive strength (e.g. smectite cement, capillary strength, etc.). This effect was studied with tests in water sensitive and insensitive sandstones.

(ii) Capillary cohesion in the near-cavity failed rock. This effect was studied with twin HC sand production tests with single-phase oil flow, single-phase water flow, and irreducible water saturation with oil flow.

(iii) Two-phase flow effects with high pore pressure gradient front moving together with the water front. This effect was studied with fast and slow water breakthrough sand production tests.

The three outcrop sandstones that were tested are the Castlegate (CG), Saltwash North (SWN) and the Saltwash South (SWS). These sandstones have a porosity of between 25-32% and an ambient Uniaxial Compressive Strength (UCS) between 1-20 MPa. They are often used as outcrop analogues for hydrocarbon reservoirs. In sand production tests, they each exhibit a different hole failure pattern, i.e. slit failure, breakout failure and uniform cavity closure [1] and thus they can be considered as representative sandstones for the study of sand production problem.