To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill string having a bottom hole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or by rotating the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional down-hole instruments, known as logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations. The drilling operator typically controls the surface-controlled drilling parameters, such as the weight on bit, drilling fluid flow through the drill pipe, the drill string rotational speed and the density and viscosity of the drilling fluid to optimize the drilling operations. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to optimize the drilling operations. For drilling a borehole in a virgin region, the operator typically has seismic survey plots which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator also has information about the previously drilled boreholes in the same formation.
Hydrocarbon zones may be tested during or after drilling. One type of test involves producing fluid from the formation and collecting samples with a probe or dual packers, reducing pressure in a test volume and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different depths or point within a single borehole. Testing may include exposing the formation or a sample from the formation to stimuli, such as acoustic energy or electromagnetic energy. From these tests, information can be derived for estimating parameters of interest regarding the formation.
Samples brought up through the borehole may become contaminated by other material in the borehole, including drilling fluid. This risk of contamination limits the value of surface analysis of the samples. Additionally, some parameters of a formation may only be estimated at the depth and under the conditions where drilling is taking place. The properties of a deeper regions of the formation (outside a mud-invaded zone) may be different from those regions in close proximity to the borehole due to the ingress of drilling fluid, which may mix with or displace native formation fluid. This contamination may result in erroneous measurements of properties of the deeper regions of the formation. There is a need for methods and apparatus for evaluating parameters of interest of a formation during the drilling process. The present disclosure discusses methods and apparatuses that satisfy this need.