Drilling Fluid Selection Method for Subsea Drilling and Its Use in Drilling Depth Calculation

Abstract

The invention relates to a drilling liquid selecting method of seabed drilling and application of the drilling liquid selecting method of seabed drilling in drilling depth calculation. In the drilling process, the densities of equivalent drilling liquid in adjacent drilling depth areas are different. The method comprises the following steps that firstly, in the drilling process, based on constant pressure P1 at the downstream tail end of an upper-layer drilling depth area Hcs, the adjacent lower-layer drilling predetermined depth H and the density rhoH of the drilling liquid pre-used within the range of the lower-layer drilling predetermined depth H, the relation of pressure P3 and the density rhoH at the predetermined depth H is obtained; secondly, due to the fact that the pressure P3 is larger than a stratum pore pressure P4 and smaller than a stratum fracture pressure P5 at the predetermined depth H, the range of the density rhoH is obtained; and thirdly, the drilling liquid which can be used is selected within the range of the density rhoH. According to the method, the situation that the pressure at a seabed wellhead is kept to be invariant is not needed, and the density selecting range of the drilling liquid is widen greatly.

Description

technical field [0001] The invention relates to the field of petroleum drilling, in particular to a drilling fluid selection method for subsea drilling. The invention also relates to the use of this method in the calculation of drilling depth. Background technique [0002] In deepwater and ultra-deepwater drilling, due to the lack of overburden pressure in the seawater section, the fracture pressure of the formation is low, resulting in a very narrow safe density range of drilling fluid. In order to adapt to this drilling environment, dual-gradient drilling methods are usually used at present. In this dual-gradient drilling method, measures are usually taken to reduce the annular pressure at the subsea wellhead to make it equal to or close to the seawater hydrostatic column pressure at the same depth (that is, the equivalent drilling fluid density at the subsea wellhead is equal to seawater density), so that the pressure change curve in the wellbore annulus can better adap...

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Problems solved by technology

[0003] However, in the above method, the pressure (or pressure) at the subsea wellhead remains constant, which leads to a relatively large limitation in the selection of drilling fluid density

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