Sedimentary rocks were initially deposited as horizontal beds and through compression were transformed into rocks. Serious forces came to bear that caused these initially horizontal rock layers to become contorted and reshaped into the layers that are seen here. This massive reshaping of rock layers can cause formation pressure to increase appreciably.
Another source of abnormal pressure is a volume of gas “capping” or residing at the top of a permeable interval. The permeable interval contains not only the gas, but possibly oil and/or water, with the fluids separating according to density.
Fracture pressure represents the pressure at which the rock fails, or fractures, and begins taking fluid from the wellbore. Contributing factors are rock type, depth of burial, and formation pressure.
Prior to drilling, fracture pressure can be determined, to varying degrees of accuracy through the following resources:
Off-set Wells – arguably the most accurate source of information – research usually yields data concerning shoe tests and the method used when the tests were conducted.
Geophysical Information – this includes seismic surveys and the interpretation of these surveys along with conclusions drawn from this information by geologists.
Field Frac Charts – in some areas, frac charts have been generated from data acquired while wells were drilled and shoes tested.
Density Log Interpretation – these electric logs can be used to estimate formation strengths by gathering data from wells in the surrounding area – the quality of the estimate is based on volume of data that can be acquired
While the well is being drilled field tests are conducted to determine the integrity of the casing shoes. These tests are:
- Leak-off Test
- Formation Integrity Test
The leak-off test, and its variations, are conducted after casing has been set, cement has cured, the shoe drilled out, and the mud conditioned. Essentially, pump pressure is applied to a static column of mud and the trend of increasing pump pressure is monitored, recorded, and used to determine at what point the formation leaked. In essence, the strength of the exposed formation is determined. Ideally, a short section (10’ to 50’) of non-permeable formation is drilled below the casing and tested. If a permeable formation is exposed, the quality of the test is compromised.
The formation integrity test is designed to apply a pre-determined stress on the cement job at the shoe based on the estimated maximum stress the shoe will have to withstand while the next hole interval is drilled.
The “traditional” Leak-Off test is performed to fail a formation. The advantage of performing this test is that the data acquired indicates at what point “something” failed. The disadvantage of the test is that something did fail with no guarantee that the failed formation will heal. This is more of an exploratory well type of test and is better conducted on younger, more forgiving formations
This test is conducted in a similar fashion to the leak-off but an effort is made to apply a stress to the shoe at least equal to what is assumed the shoe will experience as the well is deepened. The shoe will experience the Maximum Mud Weight required to get to the next TD, will also experience Surge Pressure as the drill string is tripped back in the hole during the drilling of that section, and may have to sustain the stress imposed by a Kick. Therefore, all of these are taken into account when conducting this test. Additionally, some operators test the shoe to the density of the cement to be used.
Commonly this is done:
- Estimate the max mud weight required to drill the next hole section.
- Select a numerical value representing the possible surge pressure.
- Select a numerical value representing the stress imposed by a kick.
The selected numerical values are added up to determine a Test ppg. Based on this calculated value and the mud weight in the hole at the time of the test, a surface pressure is calculated
The difference between this density and the mud density in the hole is determined and converted to applied surface pressure.