SPE-172350-MS
Determination of Effective Concentration of Chemical Treating Agents for
Cement Contamination in Drilling Fluids
Aleruchi Boniface Oriji, University of Port Harcourt; Bassey. O. Ekpo, R&D, NNPC, Ubani C.E: University of Port
Harcourt
Copyright 2014, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Nigeria Annual International Conference and Exhibition held in Lagos, Nigeria, 05– 07 August 2014.
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Abstract
The drilling fluid contamination is a major problem during oil and gas well cementation, especially when
such contamination comes from cement. The treatment is usually done with sodium bicarbonate
(NaHCO3) where pH control is not a problem but with sodium carbonate (Na2CO3) to increase the pH of
the drilling fluids with low pH. Determining the effective concentrations of these treating agents have
always been a challenge as either the drilling fluid is undertreated or over treated which could be
detrimental to the entire fluid system and the drilling operations. Therefore, there is a need to determine
the effective concentrations of the chemical agents for the treatment of cement contamination. During
cementation, the calcium hydroxide[Ca(OH)2] released by the cement decomposes and disperses calcium
ions in the drilling fluids which could lead to flocculation of the fluid system and consequently increases
the rheological and the filtration properties. When this occurs, the contaminated fluid system may be
treated with some chemical agents to precipitate the calcium ions formed depending on the severity. The
challenge remained on the determination of the effective concentrations of the chemical agents that will
optimally treat the fluid system without causing harm.
This paper investigates the effects of cement contamination on the rheological and filtration properties
of a typical KCL/Polymer Water Based Drilling Fluids and also determines the effective concentrations
of the treating agents (Sodium Bicarbonate and Sodium Carbonate) in the treatment of the cement
contamination. A fresh KCl/Polymer fluid was formulated to serve as a control standard and was then
contaminated with certain amount of class-G cement at varying proportions. The contaminated fluids were
treated with different amount of Sodium Bicarbonate and Sodium Carbonate respectively to determine the
effective concentrations required for the treatment of the different levels of the contamination. The drilling
fluids properties such as plastic viscosity, yield point, gel strength, pH and fluid loss were adversely
affected when contaminated with the cement. Experimental data were generated and linear correlations
were developed to determine the effective concentrations of the sodium bicarbonate and sodium carbonate
required for optimal treatment of specific amount of the cement contamination during cementation. This
will act as a proactive step during oil and gas well cementation to optimally treat cement contamination,
thereby reducing drilling time and minimizing overall well cost.
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Introduction
Drilling fluid is related either directly or indirectly to almost every drilling problems. This is not to say
that the drilling fluids is the cause or solution of all drilling problems but it is a major system that can often
be used to alleviate a problematic situation during drilling.. A successful oil and gas drilling operation
depends largely on the quality of the drilling fluids used. Hence accurate drilling fluids test and analysis
are necessary for proper control of their properties and to detect changes in the composition of the fluid
system. Contamination is one of the major problems facing drilling fluids during drilling operations. This
is as a result of some physical and chemical contaminants like cement, anhydrite/gypsum, Salt etc.
Therefore, a contaminant as regards to drilling fluids is any physical or chemical material that causes
undesirable changes in the drilling fluid system. Cement are by far the most prevalent contaminants
because it introduces excessive solids. The solids whether commercial or from the formation leads to very
high rheological and filtration properties and slows down the entire drilling rate (Amoco, 2006).
According to Baker Hughes (2006), Chemical treatments to remove these contaminants are possible for
some drilling fluids and impossible for others. The important rule is that treatments must match the
contaminants and results a desired effect on the fluid system. Some contaminants can be predicted and
pre-treated to control them while others may not. The predictable contaminants are cement, bad make-up
waters, massive salts, anhydrite formations, hydrogen sulfide and carbon dioxide. These contaminants can
be chemically removed in some cases before they can have an overall negative effect on the clays, organic
de-flocculants and filtration additives. Pre-treatment with the chemical treating agents have advantages as
long as it is not excessive and does not adversely affect the fluid properties or become a contaminant on
its own. Contaminants that are unpredictable and unexpected are those which results from small feed-ins
with a gradual build-up. Predicting potential contamination problems while drilling some wells is quite
difficult because most of the contaminant shows its effect by altering the properties of the system and in
part determined by the resultant properties. Therefore, It is always necessary to keep complete and
accurate records of drilling fluid properties to see the gradual onset of contamination and avoid
deterioration of the fluid system (Baker Hughes, 2006).
Contamination of drilling fluid by cement is an undesirable condition whereby the drilling fluid is made
unfit for use due to impairment of its properties by cement particles and chemicals. Cement can enter the
fluid systems from poor cementing jobs or squeeze cementing whereby calcium hydroxides is formed
leading to severe flocculation of the clays. This contamination is easily detected due to the increased level
of calcium seen as an increase in hardness, increase in viscosities and also an increase in pH. The extent
of contamination depends upon the amount and conditions of the cement. If the cement is hard and
drillable with the drilling Bits, it can be removed by the shale shaker screens but where the cement is green
and soft its severity on the drilling fluid is more and difficult to handle.
Contamination in KCL/POLYMER System
In KCl/Polymer systems, cement contamination acts to raise both the Calcium and pH which can lead to
precipitation, hydrolysis of the polymers and loss of control over the fluid system. Once this fluid system
is contaminated, it is either discarded incase of severe contamination or treated if the contamination is not
serious enough to render the entire system completely useless. Therefore the effectiveness of the treatment
is highly dependent on the severity of the contamination depending on the type of mud system, type of
contaminant, concentration of the contaminant, concentration of solids, level of pre-treatment of the mud
and probably, thinner concentrations in the mud.
The chemical removal of calcium ions with bicarbonate ions does not normally correct the damage
done to a dispersed system but rather for a non-dispersed system like the KCL/POLYMER. For a
dispersed system, It is usually necessary to treat with deflocculants to obtain desired rheological properties
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Table 1—Original properties of uncontaminated KCL/POLYMER fluid
Table 2—Properties of contaminated KCL/POLYMER fluid with 4 and 8 grams cement
Table 3—Results of treating with 1-5 gms NaHCO3 for 4 gms of Cement contamination
Table 4 —Results of treating with 1-5 gms Na2CO3 for 4 gms of Cement contamination
Table 5—Results of treating with 1-5 gms Na2CO3 for 8 gms of Cement contamination
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Table 6 —Results of treatment with NaHCO3 (8g of Cement)
and then treated with prehydrated bentonite, CMC or PAC to restore filtration and filter cake characteristics.
Materials and Methods
A KCL/Polymer inhibitive fluids system with some degree of tolerant of contaminants as a basis were
formulated, basically with, Fresh water, Potassium Chloride, Polymer, CMC starch and Caustic soda. The
KCL/POLYMER mud formulated served as reference fluid because of its level of stability in drilling most
shale formation and potentially productive sands due to its high shear thinning, high true yield strength,
improved bore hole stability, minimize clay swelling and clay hydration. Class-G cement was used to
contaminate the fluid system to alter its original properties negatively. The fluid in its original state was
first tested to determine its initial properties like plastic viscosity, yield point, gel strength, filtration loss,
pH and calcium ions. 4 grams and 8 grams of cement contamination were used and then varying the
concentration of Sodium Bicarbonate and sodium carbonate (1-5 grams ) respectively used to treat the
mud in a stepwise manner until original properties of the KCl/Polymer mud were restored. Rheological
and Filtration properties of the samples were measured using basic API mud laboratory equipment under
normal temperature and pressure.
Results / Discussion
Before the effects of the contaminants were noted, the original properties of the fluid were determined as
shown in table-1. The cement contamination actually affected the properties of the KCL/POLYMER
drilling fluid. When the drilling fluids were contaminated with the higher concentration of cement (8
grams) the fluid became highly viscous and flocculated. This resulted in increased rheological and
filtration properties etc. The addition of the treating agents gradually reduced the affected fluid properties.
For the 4 grams of cement contamination, the fluid properties decreased until the original properties of the
fluid were almost restored on the addition of 3 grams of NaHCO3. But beyond this point, the fluid
properties increased dramatically. This is because when the calcium ion got treated out by the 3 grams of
NaHCO3) the carbonate ion from NaHCO3 began to act as a contaminant indicating over treatment.
The results from the 8 grams of cement showed a more orderly pattern as more concentration of
NaHCO3 were added showing a corresponding decrease in the fluid properties. Although the original
properties of the fluid were not totally restored after the addition of 5 grams of NaHCO3 This showed that
from the decreasing trend of the fluid properties that addition of more grams of NaHCO3, above 5 grams
may have restored the fluid properties.
From the above analysis and the graphs as shown in the appendix, an approximate linear trend line
could be drawn, from which linear correlation were derived as given in equations 1–2 below.
SPE-172350-MS
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PLOTS SHOWING THE EFFECTS OF SODIUM BICARBONATE ON THE RHEOLOGICAL PROPERTIES OF THE CONTAMINATED
FLUIDS
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SPE-172350-MS
PLOTS SHOWING THE EFFECTS OF SODIUM CARBONATE ON THE RHEOLOGICAL PROPERTIES OF THE CONTAMINATED FLUID
1
where Y represent the amount of calcium ions and x represent the required amount of NaHCO3.
Ignoring the negative sign since solid cannot be measured in negative values, the equation is modified as
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2
Ideally, when a cement contamination is detected, a chemical test is performed to determine the
concentration of calcium ions, and then the above equation can be used to determine the required grams
of NaHCO3 that could be used to treat out the calcium contamination
References
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Annis, M.R. & Smith, M.V. (1996): Drilling FluidsTechnology, Exxon Company, USA,
Baker Hughes, (2006): Drilling Fluids Reference Manual,
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KarKim Drilling Fluids, Energy & Engineering Services, (2011) “Sodium Bicarbonate” Product Data
M I Drilling Fluids (1998): “Drilling Fluids Engineering Manual. Testing”.
Marquis Alliance, (2009): Bicarbonate of Soda,
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