Negative Pressure Test or Inflow Test
Wells are drilled by maintaining the required overbalance, which is the difference between the hydrostatic pressure of the mud column in the well and the formation pressure. The overbalance acts as a primary barrier against any influx from the formation. Once the well is drilled to the required target depth, it is completed for production, and the drilling fluid is displaced with lighter completion fluid to create underbalanced conditions and allow the formation fluid to flow to the surface through the completion string. If the casing or liner top packer does not have the required integrity, there may be an additional leak path that could jeopardize the production process and even pose a danger to the safety of the well.
A negative pressure test, also known as the inflow test, is conducted to test the integrity of a well. While carrying out an inflow test, the underbalanced conditions in the well are created in a controlled manner for a differential pressure to act from formation to the wellbore. The equipment configuration and process of the test are designed so that complete control of the well is always maintained. In case of any breach in the well or loss of barrier integrity, overbalance can be attained quickly without losing control of the well.
When should a Negative Pressure test or an inflow test be conducted?
Commonly, an inflow test is performed after cementing a liner. A cemented liner and top packer isolate the liner annulus from the rest of the well. The integrity of the liner top packer is important to ensure that the formation fluid does not breach and flow across the packer into the well. An inflow test is also carried out to test the cement plug or any mechanical plug placed in the well as a barrier to isolate the formation from the well. It could be well abandonment plugs or temporary plugs installed to nipple-down ‘Blow Out Preventer’ as per planned or unplanned operations. A negative pressure test or an inflow test can be carried out at any stage of the well. In any case, an inflow test should be conducted before drilling mud is displaced with lighter fluid as part of any operation in the well.