Method for Controlling Fluid Pressure in a Borehole Using a Dynamic Annular Pressure Control System

Abstract

A method for controlling formation pressure during the drilling of a borehole through a subterranean formation includes selectively pumping a drilling fluid through a drill string extended into a borehole, out a drill bit at the bottom end of the drill string, and into an annular space between drill string and the borehole. The drilling fluid is discharged from the borehole to a reservoir to clean the drilling fluid for reuse. An annular space fluid pressure is selectively increased to maintain a selected fluid pressure proximate the bottom of the borehole by applying fluid pressure to the annular space. The selective increasing includes controlling an aperture of an orifice operatively coupled between the annular space and the reservoir. A formation fluid pressure is predicted at a depth below a lowermost depth to which the borehole is drilled and the aperture is adjusted to maintain a selected pressure in the borehole in response to the predicted formation fluid pressure.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]Priority is claimed from U.S. Provisional Application No. 60 / 786,648 filed on Mar. 28, 2006.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]Not applicable.BACKGROUND OF THE INVENTION[0003]1. Field of the Invention[0004]The invention relates generally to the field of drilling boreholes using dynamic annular pressure control devices. More specifically, the invention relates to method for determining borehole fluid control events, such as loss of drilling fluid or formation fluid entry into a borehole when such devices are used.[0005]2. Background Art[0006]The exploration for and production of hydrocarbons from subsurface Earth formations ultimately requires a method to reach and extract the hydrocarbons from the formations. The reaching and extracting are typically performed by drilling a borehole from the Earth's surface to the hydrocarbon-bearing Earth formations using a drilling rig.[0007]In its simplest form, a land...

AI Technical Summary

Benefits of technology

[0020]One aspect of the invention is a method for controlling formation pressure during the drilling of a borehole through subterranean formations. A method according to this aspect of the invention includes selectively pumping a drilling fluid through a drill string extended into a borehole, out a drill bit at the bottom end of the drill string, and into an annular space between drill string and the borehole. The drilling fluid is discharged from the borehole to a reservoir to clean the drilling fluid for reuse. An annular space fluid pressure is selectively increased to maintain a selected fluid pressure proximate the bottom of the borehole by applying fluid pressure to the annular space. The selective increasing includes controlling an aperture of an orifice operatively coupled between the annular space and the reservoir. A formation fluid pressure is predicted at a depth below a lowermost depth to which the borehole is drilled and the aperture is adjusted to maintain a selected pressure in the borehole in response to the predicted formation fluid pressure.

Problems solved by technology

The fact that the drilling fluid hydrostatic pressure typically exceeds the formation fluid pressure also results in the fluid entering into the formation pores, or “invading” the formation.

Overbalanced drilling relies primarily on the hydrostatic pressure generated by the column of drilling fluid in the annular space (“annulus”) to restrain entry of formation fluids into the borehole.

However, the addition of density increasing (“weighting”) additives to the drilling fluid: (a) may not be rapid enough to deal with the formation fluid influx; and (b) may cause the hydrostatic pressure in the annulus to exceed the formation fracture pressure, resulting in the creation of fissures or fractures in the formation.

Creation of fractures or fissures in the formation typically results in drilling fluid loss to the formation, possibly adversely affecting near-borehole permeability of hydrocarbon-bearing formations.

The use of overbalanced drilling also affects the depths at which casing must be set during drilling operations.

Borehole fluid pressures in excess of the formation fracture pressure can result in the drilling fluid pressurizing the formation walls to the extent that small cracks or fractures will open in the borehole wall.

Further, the drilling fluid pressure overcomes the formation pressure and causes significant fluid invasion.

Fluid invasion can result in, among other problems. reduced permeability, adversely affecting formation production.

As noted above, while additional weighting material may be added to the drilling fluid to increase its hydrostatic pressure, such will be generally ineffective in dealing with a gas kick due to the time required to increase the fluid density at the kick depth in the borehole.

An open loop drilling fluid system is subject to a number of other problems.

It will be appreciated that these pressure spikes can cause fatigue on the mud cake and borehole wall, and could result in formation fluids entering the borehole or fracturing the formation again leading to a well control event.

The displacement of drilling fluid results in reduction of hydrostatic pressure on the annulus, further exacerbating the gas expansion in a dangerous cycle.

Images