Mr. Erik Milito - The House Committee on Natural Resources ...

ISOLATING POTENTIAL FLOW ZONES DURING WELL CONSTRUCTION 53
This document is a compilation of best practices, engineering considerations and cement property requirements to
assist in the prevention of annular flows and to establish zonal isolation within the wellbore. This task entails, at its
most fundamental level, the removal of drilling fluid from the wellbore and replacement of drilling fluid with cement
capable of achieving and maintaining annular isolation.
During the API Work Group’s study and draft RP preparation process, the relevant technology and practices
contained herein generated prolonged discussions and comprehensive work in writing the relevant text in this and
other parts of API Std 65—Part 2. Numerous literature searches were conducted to find, discuss, and cite the
information that helps document whether or not a practice is field proven, technically valid, and reliable in preventing
annular flows.
A.13 Loss of Hydrostatic Pressure After Cement Placement
<strong>The</strong> failure of an annular cement column to control and isolate zones exposed in the wellbore is the root cause for
many of the LWC incidents experienced in offshore drilling operations. LWC incidents can occur at any point in the
well construction process from soon after spudding the well to drilling the well at total depth.
A number of factors are common to LWC incidents experienced while drilling the top-hole sections. From a pressure
maintenance standpoint, many wells are drilled in a near-balanced condition. Often only a minimal pressure margin
exists between formation pore pressure and circulating hydrostatic pressure of the drilling fluid. Typically, the well is
drilled with simple spud muds with minimal fluid loss control. <strong>The</strong> cement designs employ lightweight, extended lead
cement systems with a tail cement of higher density placed in the lower section of the cemented interval. In common
practice, both lead and tail cement slurries are designed without any gas control capabilities. Further, certain
lightweight and other cement systems are prone to gel before setting, thereby causing and accelerating the loss of
hydrostatic pressure exerted on the column of tail cement below. A detailed discussion of this phenomenon is
included in A.13 and A.14 where the “loss of hydrostatic pressure” effect can be caused by pre-mature gellation, also
called early static gel strength development of the “critical gel strength period” (see 5.7.8) [18] .
Annular flows have been caused by hydrostatic pressure losses that occur before the cement cures into a hard,
impermeable barrier. This has happened in both top-hole sections and bottom-hole sections of the well. Several
factors or combinations may cause annular flows in the deeper sections of the well including the cementing process,
cement design, and the immediate setting of mechanical barriers that can reduce hydrostatic pressures.
While mechanical barriers are designed to prevent the flow of annular fluids past the barrier element or seal, setting of
the barrier may actually increase the chance of gas entering the cement slurry. This is because setting the barrier
isolates all potential flow zones below the barrier from all of the hydrostatic pressure above the barrier. This reduction
in OBP on any potential flow zones effectively decreases the CSGS as defined in 5.7.8. <strong>The</strong> pressure in the annulus
therefore drops to the pore pressure of the flow zones at an earlier time after the cement is in place, increasing the
window of opportunity for gas to enter the cement slurry. Because of this increased chance of gas entering the
cement, it is very important that the slurry placed across potential flow zones is designed with gas migration control
properties (see 5.7.13). Properly designed cement slurries should be used to help prevent the gas from migrating
through the annulus once it has entered the cement. If migration is not controlled there is potential for either a crossflow
into a lower pressure zone or the collection of a gas pocket directly below the mechanical barrier.
In other cases (no mechanical barrier), annular hydrostatic pressure losses may fall below permeable formation pore
pressures resulting in underbalanced conditions that cause higher pressure formation liquids and gases to flow into
the cemented annulus. This can lead to annular flows of formation liquids and gases which may induce cross-flows
into permeable formations with lower pore pressures, paths that flow up to the wellhead, or a combination of both.
Cooke et al [24,25] investigated the loss of hydrostatic pressure in columns of drilling fluid and cement slurries and
reported the results in SPE papers 11206 and 11416 and in JPT articles dated August 1983 and December 1984.
Cooke studied hydrostatic pressure losses by measuring annular pressures vs time at various depths with sensors
installed on the casing and hard wired to surface recorders. Measurements were recorded prior to, during, and after
primary cementing operations in several wells. Measurements were recorded for several months in some wells that
showed long term reductions in drilling fluid column hydrostatic pressures.