(156g) A Model-Based Approach for Predicting Gas Well Leaks from Cemented Annulus and Identifying Well Construction Factors Responsible for Such Leaks

A Model-based Approach for
Predicting Gas Well Leaks from Cemented Annulus and Identifying Well
Construction Factors Responsible for Such Leaks

Misra Shobhit and Nikolaou Michael

            Gas leaks from
natural gas wells can create severe environmental and safety problems, as the
leaked gas may end up in the air or water. 
Poor cementing job and other well completion operations of gas wells may
allow gas from one rock formation zone to migrate into another and eventually
leave the well.  Therefore, it is
important to ensure that natural gas well construction prevents such unwanted
events.  The design objective is known as
zonal isolation and has been the subject of intense study recently.

            In well construction
jobs that do not ensure zonal isolation, natural gas may move through the small
channels created in the cement sheath in the annulus between the well casing
and well wall, and may eventually reach the well head.  To detect whether a continuous flow of gas
reaches the wellhead a simple procedure is followed, namely a pressure gauge
followed by a needle valve are installed, and the needle valve is temporarily
opened to lower pressure by bleeding off a small amount of gas, if pressure
increase is detected by the pressure gauge. 
If gas pressure builds up again after the gas bleed off and closing of
needle valve, there is clear indication that there is communication between the
well head and producing sections of the well through the cemented well wall.  The resulting pressure is called sustained casing pressure (SCP) and is
undesirable.  The objective of this work
is to determine what factors contribute toward SCP, and prevent such factors
from creating measureable SCP values. 
Because the effect of all relevant factors on SCP is complicated, the
proposed approach relies on statistical analysis of a large volume of
data.  Through such analysis, a model is
built that captures quantitatively the effect of related factors on SCP.  These factors are associated with well
drilling, cementing and hydraulic fracturing parameters (Nelson 2006).

            A
statistical technique called partial least squares (PLS) regression was used to
create the model.  The model structure
involves a number of latent variables, which are linear combinations of the
original variables, and which are determined through leave-one-out
cross-validation in such a way, that correlation between model inputs and
outputs is maximized.

Variable
Importance in Projection (VIP) variable selection method was used to exclude
the less relevant variables.  As the term
suggests, VIP score of a variable indicates the contribution of a particular X
variable to the predicted value of Y (SCP in this case).  In our model we excluded the variables with
VIP scores less than 0.5.

            The
contribution of various variables to SCP is shown in the bar chart (Figure 1).

Figure1. VIP
scores for variables

References:

Nelson, E. B. (2006). Well Cementing.
Texas, USA.

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