The pressure integrity test is a test in which the drilled hole is pressurized to a certain level of surface pressure. The test is conducted to ascertain or confirm the resistance of the drilled formation.
When planning for the mud program, it is useful to know the maximum mud density which can be used during drilling operations. The maximum mud weight is limited by the fracture gradient which can be defined as the minimum In-Situ stress. In order to avoid getting to mud losses or formation fluids influx, the mud weight must lie between the pore pressure and formation fracture gradients.
The fracture pressure can be defined as the required pressure which can induce fractures in the formation at a given depth. It can be also defined as the pressure which the injection of the fluids cause the rock to fracture hydraulically. The orientation of the produced fracture depends on the orientation of the principal stress of the fractured point. At any point of the formation, there are three perpendicular stresses. The fracture will be developed perpendicular to the minimum stress. The fracture can be initiate when the borehole pressure is greater than the least principal stress and it will propagate when the surface pressure is maintained at a pressure level greater than the least principal stress.
The formation integrity test term involves various meanings, it can includes:
- The limit test which carried out to specified value below the fracture gradient
- Leak off test which is carried out to point where the formation starts to leak off
- Fracture gradient test which is carried out to leak off point and beyond until the formation around the well bore fails.
There is another term which is widely used in the oil field, it is the shoe bond test which is carried out to test the strength and the integrity of the cement at the casing shoe.
The fracture gradient at the well planning stage can be estimated from the offset well data. If the offset data is not available, the fracture gradient can be estimated using the theoretical models.
During the drilling operations, the formation pressure test (Shoe bond test) can be carried out to determine approximately the fracture gradient. The test pressure can be converted to an equivalent mud weight which can be considered as the upper limit of the primary well control barriers for the next hole section.
The formation integrity test objectives
The formation integrity test is conducted for many reasons:
- To investigate the strength of the cement around the casing shoe and to ensure that there is no communication with higher layers of formation.
- To estimate the fracture gradient in order to determine the upper limit of the mud weight which is the primary well control barrier.
- To collect information to use them for the optimization of the well design for future projects
- The information gathered from this test can be used to select the casing seats
- It can be used also for planning the hydraulic fracturing.
Sub-Surface stresses
The resultant stress exerted at any particular point beneath the earth's surface can be resolved into three stresses which are perpendicular to each other: vertical stress Qv and two horizontal ones QH and Qh. These stresses are described as principal stresses and they are designated as follows:
- The maximum principal stress ( σ1)
- The intermediate principal stress (σ2)
- The minimum principal stress (σ3)
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Fig 1: Presentation of the In Situ-Stresses |
In most cases the maximum principal stress is the vertical stress due to the pressure exerted by the overlaying rock, and it is called the overburden pressure.
The formation fractures when the bottom hole pressure is greater than the minimum principal stress. The fracture propagate along the perpendicular plan to the direction of the minimum principal stress.
During drilling operations the in-situ stresses are disturbed by the creation of the wellbore and it can induce more stresses around the wellbore which can strengthen that region.
When testing the formation by pressurizing the wellbore, a graph of pressure vs a pumped volume can be established. For the first section of the graph, initially a linear trend can be noticed. If the pressure is increased further, then theoretically the formation will fracture when the pressure reaches the minimum horizontal stress. However, the formation around the wellbore is strengthen by the stresses created during the drilling operations, so the formation fractures at a pressure higher than the horizontal stress. Once the fracture is generated at the rock, the required pressure to maintain the fracture is less than the pressure used to create the fracture.
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Fig2: Qualitative graph of the formation integrity test |
The description mentioned above is for the vertical wells. In deviated wells, the fracturing pressures can be different due to many reasons and it is related to the hole inclination and azimuth.
Similar to the deformation of solid materials, the rock during the pressurizing process can response differently according to many factors. Some formations have elastic behavior and others have plastic behavior. The level of the stress needed to cause the formation to deform elastically, plastically or fails depends on the rock strength, permeability and mineralogy. In some cases, the formations have natural fractures which are developed in response to the tectonic stresses. Due to all these factors, the fracture gradient can change significantly.
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