Wellbore fluids for cement displacement operations

Abstract

A method of cementing a pipe into a wellbore filled with a drilling fluid that includes displacing the drilling fluid with the displacement fluid which includes a base fluid, a micronized weighting agent; suspending a pipe in the wellbore; and pumping cement into the wellbore to substantially fill the annulus formed between the outer surface of the pipe and the wellbore is disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation-in-part of co-pending U.S. patent application Ser. Nos. 11 / 737,284 and 11 / 737,303, which are, respectively, a continuation application and a divisional application of U.S. patent application Ser. No. 10 / 610,499, which is a continuation-in-part of U.S. application Ser. No. 09 / 230,302, which is the U.S. national phase application under 35 U.S.C. § 371 of a PCT International Application No. PCT / EP97 / 003802, filed Jul. 16, 1997 which in turn claims priority under the Paris Convention to United Kingdom Patent Application No. 9615549.4 filed Jul. 24, 1996. This application is also a continuation-in-part application of co-pending U.S. patent application Ser. No. 11 / 617,576, which is a continuation application of U.S. patent application Ser. No. 11 / 145,054, now U.S. Pat. No. 7,176,165, which claims priority to U.S. Provisional Application Ser. No. 60 / 576,420. This application is also a continuation-in-part appli...

AI Technical Summary

Problems solved by technology

In some cases, the rheology of the drilling fluid being displaced may be such that the pressures in the wellbore while pumping the cement slurry may exceed the fracture pressure of the formation.

This undesirable event may result in significant fluid loss to the formation, and consequently a significant increase in cost due to increased wait time or remedial repair.

While cementing efficiencies and zonal isolation may be realized with proper drilling fluid displacement, cementing generally requires for a much narrower hydraulic tolerance upon the borehole, thus restricting pump pressures and high flow rates.

If the ECD of the drilling fluid exceeds the ability of the formation to resist fracture, fluid losses (also referred to as lost circulation events) typically result.

Additionally, due to narrow fracture formation pressure, little room is left for conventional ECD reduction devices, and thus, ECD must be controlled by tailoring the rheological properties of the drilling fluid.

While it is highly advantageous to maintain the wellbore pressures above the pore pressure, on the other hand, if the pressure exerted by the wellbore fluids exceeds the fracture resistance of the formation, a formation fracture and thus induced wellbore fluid losses may occur.

Therefore, a major constraint on well design and selection of wellbore fluids is the balance between varying pore pressures and formation fracture pressures or fracture gradients though the depth of the well.

A particularly challenging situation arises in depleted reservoirs, in which high pressured formations are neighbored by or inter-bedded with normally or abnormally pressured zones.

This can make the drilling and completion of certain depleted zones nearly impossible because the wellbore fluid weight required to support the shale exceeds the fracture resistance of the pressure depleted sands and silts.

If settlement or sag occurs, an uneven density profile within the circulating fluid system, and thus well control (gas / fluid influx) and wellbore stability problems (caving / fractures), may result.

High rheology properties can result in an increase in pressure at the bottom of the hole under pumping conditions.

Increases in ECD, as mentioned above, can result in opening fractures in the formation, and serious losses of the wellbore fluid into the fractured formation.

The goal of low rheology fluids with low viscosity plus minimal sag of weighting material continues to be a challenge.

Drilling fluids that are viscous, with high gel strengths are particularly problematic to the ultimate success of a cementing operation and in some cases, as mentioned above, it is necessary to reduce the rheology of the drilling fluid prior to the cementing operation.

However, reducing the rheology of a drilling fluid may induce settlement of the drilling fluid weight material which may then be difficult to remove by the cementing fluid, cause channeling and ultimately an incomplete bond between the cementing fluid and casing.

Thus, one requirement of these wellbore fluid formulations is that the additives therein form a stable suspension and do not readily settle out.

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