API Fluid Loss Test (low-pressure, low-temperature filtration test) is a test to measure a filtration of mud with ambient temperature and 100 psi differential pressure. The API testing equipment is shown below.
How will you test the drilling mud?
• Place a filter
• Add the sample in the testing chamber
• Place the chamber in the testing kit
• Apply 100 psi pressure
• Record volume for 30 minutes at the end of the test the volume of filtrate will be recorded.
• Record thickness of filter cake
If the drilling mud has good fluid loss property, you will get a thin and an impermeable mud cake. Please keep in mind that this test is based on the surface condition, and it may be error because it does not simulate downhole conditions. The API fluid loss test can lead you to the wrong conclusion because at the surface condition, the test demonstrates very good fluid loss and very thin filter cake. When the drilling mud is in downhole condition, wellbore temperature and pressure will change the fluid property dramatically. The best way to test the fluid loss is to simulate wellbore temperature and pressure, and it will be discussed in the HPHT fluid loss later.
HTHP Fluid Loss Test
First of all, I would like to tell you about what HTHP stands for. HTHP stands for High Pressure High Temperature. The HTHP test is similar to the API test because it tells us about drilling mud filtration into the formation under a static condition over certain period of time. For the HTHP test, you can vary both temperature and pressure to represent your expected downhole condition. The HTHP testing equipment has a heating jacket so you can heat up the drilling fluid sample to the expected wellbore temperature. Typically, the recommended temperature in the heating jacket should be above the estimated temperate about 25F to 50F. You don’t want the testing temperature too high or low because it will not represent downhole condition very well. Therefore, with the recommended temperature, the drilling mud will not be over or under treated. Moreover, while performing the test, it must be under pressure and the test should be conducted at 500 psi differential pressure. The HTHP test is performed for 30 minutes, like the API fluid lost test.
The impacts on drilling operation are listed below:
Formation damage – If the drilling mud does not have good fluid loss property, fluid with small particles in drilling mud can be invaded into formations causing the formation damage. If the well is severely damaged, it will not be able to produce after perforation. This situation will heavily affect the profit of oil companies.
Differential sticking – The drilling fluid that has bad fluid loss will form a very soft and thick mud cake across the permeable formations. It can lead to differential sticking incident because the contact area between formation and drill string is increased.
Torque and drag – A thick mud cake across porous zones can be easily formed because the drilling mud has high fluid loss values. The thicker of mud cake is, the more torque and drag are experienced while drilling and tripping operation.