Polymer Mud System in Drilling Engineering


Any mud system using linear water-soluble polymer as treatment agent can be called polymer mud system. Polymer drilling mud has been developed as a new drilling fluid system since the early 1970s.  But in the process of application, polymer mud system usually refers to mud system in which the polymer is the main treating agent or controls the performance. Polymer mud was initially developed to improve the drilling efficiency. Acrylamide (PAM), a polymer flocculation agent, was firstly used in the drilling mud as early as 1958. PAM was called total flocculent because it can flocculate drilling cuttings and bentonite. A little PAM added in mud can drive the drilling cuttings out of the well. Partially hydrolyzed polyacrylamide (PHP) polymer was found in 1960, which has selective flocculation character, flocculating low-quality clay and cuttings, other than flocculating mud-making clay with high quality, meanwhile, inhibiting the dispersion of drilling cuttings. The amount of submicron particles in mud system treated with PHP is significantly lower than other types of water-based drilling fluid, which is beneficial to improve the drilling speed. The new mud system is called polyacrylamide non dispersed drilling mud of less solid phase.

The polyacrylamide non dispersed drilling mud of less soildphase was first used in 1966 by Pan Am, which
greatly improved the drilling rate. And then it was spread in the world and tested in different strata, well depth and
density. In addition, it has obvious advantages in the case of drilling speed improving and cost saving. As a mud
system equipped with advanced technology, the polyacrylamide non dispersed drilling mud of less solid phase was
continuously adopted by slim-hole drilling in the coal field, geologic engineering, metallurgy and nonferrous
geology. However, some weaknesses also exist: (1) as a selective flocculant, the optimal quantity range is relatively
narrow. The floccule is sensitive to concentration, for the structure is loose. The flocculation effect will become
worse with high concentration. Especially meeting with water sensitive strata containing plenty of montmorillonite,
the flocculation effect is poorer; (2) Mud system network structure is too strengthened and the static shear force is
over-large, which can easily affect the rheology.

In the recent 20 years, polymer treating agent has got a rapid development in research, production and application,
and the types for different functions become more and more various. For example, polyhydrolyzate (Na-HPAN),
potassium polyacrylate (NPA or NH4-HPAN), compound ionic polyacrylic acid (PAC141), potassium polyacrylate
(PAC141) and potassium polyacrylate (K-PAM) have been widely applied in the polymer mud system, and
expanded to other areas.

Characteristics and Main Function Mechanism of Polymer Mud

  1. The Characteristics of Polymer Mud

Laboratory experiment and field application show that polymer mud has the following characteristics compared with other water-base mud:
(1) Moderate mud density, low solid content, and less proportion of submicron particles mainly result from selective flocculation and the inhibition of the rock cuttings dispersion of polymer treating agent.
(2) Having favorable rheological property, which mainly showed better shear thinning behavior and appropriate flow pattern. The structure of polymer mud system was formed by the interaction between particles, the bridging role between polymer molecules and particles and the interaction between polymer molecules as well. The structural strength mainly depends on the strength of the bridging role between particles and polymer molecules. Under the condition of high shear stress, the bridging role is destroyed so as to reduce the viscosity and shear stress, which presents better shear dilution effect and stronger thixotropy for suspending and discharging rock cuttings. With high shear rate around bit nozzle, the lower fluid resistance is beneficial to improve drilling rate.
(3)Polymer treating agent is conducive to keep the borehole stable, for the functions of good presidium and dispersion inhibition. Therefore, this type of mud has been widely applied at home and abroad since the 1970s, and gained rapid development in technology.
During the practices of slim-hole drilling in the coal field, geologic engineering, metallurgy and nonferrous geology, polymer mud system was mainly composed by ammonium hydrolyzed polyacrylonitrile (NPAN/NH4- HPAN, usually known as ammonium salt) and potassium polyacrylate (KPAM). Ammonium hydrolyzed polyacrylonitrile is formed from polyacrylonitrile and ammonium hydroxide in the condition of high temperature and pressure. Its relative molecular weight is about 80110 thousands and degree of hydrolysis is about 50~60%.
It’s a high temperature resistance filtrate reducer, and does not increase viscosity. In addition, it is also a good anticollapse agent, for the NH4+ provided can restrain the hydration dispersion of clay and shale.
Potassium polyacrylate is a sylvite of partially hydrolyzed polyacrylamide that is a ramification of polyacrylamide. And the weight of potassium polyacrylate added between 0.1% and 0.3% is usually enough to stabilize the wellbore.


2.The function mechanism of polymer mud system

1.Mechanism of Anti-sloughing

Treatment agents of polymer mud are mainly high molecular polymers with polar functional groups, which can adsorb water molecules. The polymer sticks to clay particles surface through hydrogen bonding or electrostatic
attraction, when the polymer mud contacts with the rock around the borehole. Because of high molecular weight and
good linear expansion ability, the polymer can not only stick to a clay particle but also further connect adjacent clay
particles, which can make multiple clay particles connected or bridged together. Therefore, the polymer mud could
prevent the rock dispersion and keep the stability of borehole.
The hydration energy of K+ and NH4+ are 393 KJ/mol and 364 KJ/mol, respectively. Both of them are lower
than that of Li+, Na+, Ca2+ and Mg+. Due to the selective adsorption of clay particles for cations, especially the
cations with lower hydration energy, K+ and NH4+ adsorb rock in the vicinity of borehole preferentially. In addition,
the diameters of K+ and NH4+ are 2.66 A0 and 2.86 A0 respectively. The size just makes it possible that K+ and
NH4+ can enter into silicon-oxygen tetrahedron-hexagonal network of clay mineral (the diameter of hexagonal
network is about 2.6 A0 ), and water is gradually squeezed out from the upper and lower layers of clay with
increment of the adsorption degree, thereby reducing the hydration of clay. Because of the less radiuses of hydrated
K+ and NH4+ ions, they can embed in illite layers (the gap between the layers of illite about 10.6 A0), while other
hydrated ions (except Cs+ and Rb+) can’t in consideration of greater radius. When K+ loses adsorbed hydration
shell, the ion radius becomes slightly smaller, and the tetrahedral structures of adjacent crystal layers are close to
each other. As the process continues, the contraction effect forces K+ into the free space of the surface of exposed
rock near the borehole, then is quickly fixed in position, thereby limiting the expansion and the separation of
adjacent silicate chip, this process is called the lattice fixed effect of K+. As a result of this effect, it makes K+ as
close as possible to the negative charge center between adjacent crystal layers and leads the formation of compact
structure which is almost impossible to strongly hydrate again in the water. Because this structure has been
dehydrated and compressed during formation, therefore it is relatively stable and hard to be replacement by K+.

2.Function of De-filtration

Polymer treating agents maintain the existence of a certain amount of fine grains in the mud mainly by bridging role and forming stable space grid structure with clay particles, and can form compact mud cake on the borehole wall. With strong hydration ability amide ion groups (CONH2), fluid loss additive molecules can increase the thickness of hydration film. Moreover, polarized water of high viscosity in the mud cake can effectively prevent free water from infiltrating the strata near the borehole wall. Fluid loss additive can improve filtrate viscosity and plug the micro cracks in the mud cake and strata near the borehole wall, resulting in more compact between the mud cake and borehole wall rock covered by itself and the decrement of fluid loss.


Field Application

Geologic Situation and Characteristics

Geothermal wells are mainly near the fault zone with complex structure and fracture development, most of which will pass through the clay, sand and gravel layers of quaternary and the formation of mudstone and sandy conglomerate etc. of tertiary. Geothermal well drilling is usually with a high drilling rate and large caliber, and most of the well in the bottom is 152 mm open hole section (refers to the aquifer), which means the mud with low solid content, medium density, low viscosity, low fluid loss, good rheology and wall building ability (low fluid loss and thin and resilient mud cake, etc.) is required. This mud system should have a strong ability of inhibiting hydration dispersion of cuttings and shale near the borehole well, thereby getting the function of anti-sloughing, leak-proof, anti-shrinkage and preventing fall-block. In addition, it can suspend and exclude cuttings effectively to ensure quickly and efficiently drilling.

Geologic characteristics of coal exploration area can be acquired from the example of Song Jiaying mining area in Tangshan: existing thick alluvium, high mud making, unstable formation and the problems of easy to collapse, fall-block and borehole shrinkage, resulting in it difficult to explore. The designed depth of prospecting hole is about 900 ~ 1200 m, among of which the Cenozoic alluvium is very thick and the average thickness is more than 800 m. In addition, the quaternary alluvium is mainly comprised of gravel, coarse and fine sand, silt and sandy clay. These formations have poor cementation so as to easy to collapse and hole shrinkage. There are many downhole accidents such as sticking, burying drilling and hole deviation during the past drilling.

The main lithologies of coal measure strata are mudstone, siltstone and coarse gravel sandstone. The thickness of coal bed is almost up to 10 m in the case of extreme degree. Mudstone can easily lead to mud making and hole shrinkage, meanwhile, coal bed can easily lead to collapse and hole shrinkage. The borehole stability is poor using conventional drilling mud, which can often result in downhole accidents such as sticking and burying drilling due to the borehole instability. And this has caused great difficulty in construction and low drilling efficiency, and further
impacted the economic benefit.

Basic Composition and Relative Performance of Polymer Drilling Mud System

  • The basic composition of polymer drilling mud system
    (1) Large-diameter geothermal wells
    Base mud after pre-hydrated for 24h (4~6% bentonite or peak clay + 0.3~0.5% soda ash + 0.05~0.1% caustic soda) + 0.3~0.5% NPAN + 0.05~0.15% KPAM + 2~3% modified bitumen (FT342)
    Relative performance:
    Funnel viscosity: 20~35s; density: 1.05~1.25 g/cm3; filtration: 4~12ml/30min; sand content: ≤0.5%; cake
    thickness: ≤0.5mm; pH:8~9.
    (2) Coalfield drilling
    Base mud after pre-hydrated for 24h (8~10% peak loess + 0.4~0.5% soda ash + 0.1%caustic soda) + 0.3~ 0.4%NPAN + 0.050.1% KPAM.
    Relative performance:
    Funnel viscosity: 20~25 s; density: 1.05~1.15 g/cm3; filtration: 8~12 ml/30min; sand content: ≤0.5%; cake
    thickness: 0.5~1 mm; pH: 8~9.
    (3) Wire-line drilling
    On the basis of the traditional water-based bentonite mud, it is formulated into a low solid polymer mud with
    0.3~0.4% NPAN and 0.05~0.1% KPAM.
    Relative performance:
    Funnel viscosity: 20~25s; density: 1.05~1.10 g/cm3; filtration: 5~10 ml/30min; sand content: ≤0.5%; cake
    thickness: 0.5~1 mm; pH: 8~9.
  • Control and maintenance of mud
    KPAM is used to be a selective flocculant, with amount of 0.1 to 0.3%, so as to reduce filtration qualities and
    increase viscosity. 0.3~0.5% hydrolyzed polyacrylonitrile potassium (K-HPAN, commonly known as potash) is
    added as dispersed shale hydration inhibitors and also can reduce filtration qualities as well as improve temperature-tolerance up to 150~180 °C. In addition, 0.3~0.5% hydrolyzed polyacrylonitrile ammonium (NH4-HPAN, commonly known as ammonium salt) has the same ability as K-HPAN. During drilling, the condition of the underground is evaluated according to the data showed by drilling instrumentation. And then adjust the drilling fluid flow patterns and performance indicators by changing the content of the polymer to better meet the requirements of drilling.
    With the increase of hole depth, the sections of unstable location (easy to collapse, hole shrinkage) increase, then we must increase the amount of dispersed shale hydration inhibitors and fluid loss agent. When drilling in the area easy to collapse, increasing the density of mud through adding in 23% asphalt products is beneficial to keep the stability of borehole.
    When drilling in the area easy to leakage, increasing the viscosity of mud by increasing the potassium polyacrylate so as to control filtration reasonably, while sealing cracks in the formation with inert materials such as sawdust.

Field Application Analysis

From 2001 to 2003, the polymer muds mentioned above are successfully applied in drilling operation of Geothermal Project JR118 (3605 m) and JR130 (3702 m) in Beijing, and have achieved good results. The result is shown in Tab. 1.

Table 1. The Situation of Drilling Operation of Geothermal Project JR118 and JR130

well number JR118 JR130
well depth /m 3605 3702
actual drilling time /h 1539 1632
 well-washing time /h  30  32
 construction cycle/d  150  150
 artesian discharge /m3·d- 1  1500  1500
 the cost of drilling fluid /yuan·m-1  40  50
 reduction rate of cost /%  21  18
 note geothermal well geothermal well

As is shown in Table 1, the polymer mud which is applied in drilling operation of Geothermal Project JR118 and
JR130 can significantly reduce the drilling cost and is helpful to improve the comprehensive economic benefit of

In addition, the application of polymer mud in coal field exploration district 9-1 hole in Tangshan has been evaluated in 2002. Adjusting the polymer treating agent timely according to the stratigraphic characteristics of different depth prevents the potential drilling failure, and eventually smoothly passes through alluvium to reach the target formation. Subsequent evaluation at the scene shows that the polymer mud has strong ability of shale hydration inhibition. It can form thin, toughening and smooth mud cake. In addition, it has good lubricity and strong ability to carry rock flour, which can solve the problems such as collapse, hole shrinkage and leakage in the area of clay and sand layers to improve the drilling efficiency, reduce the labor intensity of the workers, and save the drilling cost.

From 2004 to 2006, polymer mud has also been applied in 11-1,12-2 and 5-1 hole in Tangshan coal field
exploration, and the result is satisfactory (as shown in Tab. 2).

Table 2.The situation of drilling operation of Tangshan coal field exploration

No. 9-1 11-1 12-2 5-1
 Depth (m)  943.33  842.79  916.97 980
 Average hourly efficiency(m·h-1)  1.65  1.83  1.75  1.77
 Drilling efficiency per month(m·month-1)  400  486  811  1088
 Construction period(d)  70  54  34  27
 Note Coal exploration wells

5. Conclusions
Practical application shows that polymer mud has the following characteristics:
(1) The mud has strong shear thinning performance, good rheological and suspension aggravated ability, and is
easy to control in real-time during the process, which can apply rapid high-pressure jet drilling technology.
(2) With good flocculation ability to carry rock powder, lubrication performance of drag reduction, the inhibition
of shale hydration and the function of anti-sloughing as well as wall building property. And good effect of antisloughing has been showed in the quicksand, water sensitive formation and coal bearing strata. In addition, the trip was operated successfully and the electric logging can be completed one time.
(3) It is conducive to the protection of aquifer. Polymer mud has the characteristics such as low solid content,
moderate density, strong bridging-off performance, and thus can protect the aquifer effectively. Aquifer is soaked by flushing fluid for 24h after finishing drilling and mud displacement. Then drill down and swill the aquifer with clear water. Gas lift reverse circulation with air compressor for 6h, then pump water without hydrochloric acid rinsing, which can shorten the time for washing wells and testing pumping operations.
(4) It is beneficial to reduce the cost of drilling mud. Compared with ordinary dispersed mud, polymer processing
agent can greatly save cost for preparing mud, because the component of agent such as ammonium salt and
potassium can be recycled.

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