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some drilling fluid produces hydrogen sulfide under the
action of high temperature.
Hydrogen sulfide is a gas of colorless, toxic, strong
acid. Hydrogen sulfide gas in low volume fraction has a
smell of rotten eggs, the relative density of which is 1.19.
Compared with air, it is heavy, and it can dissolve in
water. When burning, it accompanied by a blue flame and
produces sulfur dioxide gas that is very harmful to the
person. The mixture of hydrogen sulfide and air, while
mixture gas volume fraction lie the range of 4.3% ~ 45% ,
form a kind of explosive mixture. Hydrogen sulfide is
easy to gather, not easy to drift, often gathering at the
bottom of drilling platform or the lower well site. It can
react with many metals and can corrode of metal severely.
When volume fraction of hydrogen sulfide gas in the air
is greater than 10 × 10-6 , people feel discomfort, and
when that is more than 20 × 10-6, may cause poisoning.
In general, volume fraction of hydrogen sulfide
increases with increasing formation depth. The presence
of hydrogen sulfide is in two forms, either in the form of
hydrogen sulfide gas alone or coexisting with oil and gas;
or in the form of dissolving in water and oil. In drilling
process, the hazards of hydrogen sulfide can be summed
up in four aspects: 1 harm on the human body; 2
corrosion of drilling equipment; 3 contamination of
drilling fluid; 4 pollution of the environment[3].
B. Engineering factors
Geological factor is the objective reality and can not be
changed. If we have better understanding about it and
take corresponding countermeasures, we can turn danger
into safety, reduce the complexity of drilling, and avoid
the accident. Engineering factors is a specific
manifestation of subjective factors, that is construction
design, technical operations, craft measures and so forth
and is suitable for geological factors. It is human factors,
modifiable and adjustable. For engineering factors, see
Table1.
III. COMPREHENSIVE EVALUATION OF DRILLING RISK AND
MODEL BUILDING
Comprehensive evaluation of drilling risk is that the
organization of professionals, based on existing
professional experience, evaluation criteria and
guidelines , identify possible risks or impacts at predrilling
activities, evaluate and analyze scientifically,
calculate the probability ,value and consequences of the
risk by quantifying risk factors, and thus determine
whether the project risk may be accepted. Comprehensive
evaluation methods mainly are analytic hierarchy process,
decision tree method, monte-carlo method, fuzzy
comprehensive evaluation etc. In this paper,
unascertained measure evaluation method is adopted for
analyzing and processing, and its evaluation results can
be quantified accurately. The specific process is as
follows:
A. Establish hierarchy structure of drilling risk
Establish hierarchy structure of risks by risk
identification, like Figure1. Target layer denotes the
object evaluated. Criterion layer is factor categories that
evaluation object contains, that is two elements of drilling
risk assessment. Each element is a relatively independent
entity, which can be used as an object for study. Index
layer is indicators that various factors included.
B. Determine the weight of each factor
In drilling project, the importance of each evaluation
factor is different, therefore we should determine their
weighted coefficient according to relationship between
primary and secondary to risk effect. In this paper, AHP
method is adopted to determine the weight set of
evaluation factors, that is to have experienced experts to
make a comparison with the importance of two indexes to
construct a consensus judgment matrix, then to get the
weight value using the group decision-making coordinate
method.
C. Establishment of unascertained measure evaluation
model
In comprehensive evaluation of drilling engineering,
analyzing and evaluating risk factors of system
quantitatively and intuitively is an important part in
theory circle. As drilling process involves a lot of
uncertain information, use unascertained mathematics
dealing with this issue.
1) Single-index measure modle of drilling risk
assessment:In Figure1, X indicates object space,then:
X =( x 1
, x 2
), thereinto x 1
, x 2
indicate geological factors
and engineering factors respectively. To evaluate the
object need to identify m indexes, if x indicates
ij
measured value of object x with regard to index
i
I , then
j
x can be expressed as m dimensional vector:
i
x i
=
( x , x , L , x ) ;then x1 = ( x11, x12, x13)
, x2 = ( x21, x22, L , x28)
.
i1 i2
im
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