ADVANCED PROCEDURES FOR THE DIFFICULT WELL` CEMENTING

THE STATE OF THE WELL

The state of the well is described by the next correlation:

pp <= pem <= pn <= pf (1)where:

pn is the hydrostatic pressure of fluid from the well at a certain depth;

pn = hρng (1a)

pp – the pressure of fluids from the well walls rock pores, at a certain depth;

pp = hГp (1b)

pf – the rock fracturing pressure, at a certain depth;

pf = hГf (1c)

pem – the mechanical equilibrum pressure, when the least principal effort is radial one.

the meaning of terms is:

h – the current depth;

ρn – the drilling fluid density;

g – gravitational acceleration;

Гp / f – the pressure rate from pores, respectively for fracturing the rocks.

LIMITROPHE ROCKS DAMAGE

Because the well pressure is greater than pore

pressure, drilling fluid seepages through well` wall and affect the flow capacity of limitrophe rocks.

This affecting is named damage, or skin and consist thereof complex of phizico-chemical phenomena which take place at contact between invaders fluids and particles and reservoir fluids and solids.

THE EXTENT OF DAMAGE

Lost pressure in skin zone

THE PHENOMENA THAT TAKE PLACE IN CEMENT PASTEThe Hydration

- Tricalcic silicate

- Tricalcic aluminates

• The structuring

• The Hardening

Tricalcic silicate• Tricalcic silicate through hydration forms hydrated calcium silicates -of the kind of C-S-H in

diverse structural types (alpha, beta, gamma), depending on the increase of the value of the rapport of C/S from 1 to 2 or of increase in C concentration in solution- and portlandit (calcium hydroxide) symbolized C-H, that can exist in solution both in a crystal and amorphous form. If the portlandit has a lamellar form being formed through edges to edge bonding in the same plan of the dioactahedron of calcium, the C-S-H microcrystal have a more complex structure, from here the capacity to fix more water. A C-S-H microcrystal of alpha type is formed from 2-4 parallel lamellas, spiralled together to form a tubular structure. A lamella is similar with that of montmorillonit- is triple stratified: a layer of calcium dioctahedron is placed between 2 layers of tetrahedron of silica, with the tops directed towards octahedron and the planes of bases towards exterior. The water is found in 4 forms in their relation to the C-S-H micro crystals and specifically interchristalin free water; water fixated on the extern surfaces of the crystals on the layers of the tetrahedrons, structured in hexagonal network-accordingly with the tetrahedron’s distribution- oxidrilic or polar bonded in several parallel layers; water intrachristaline fixated polar between lamellas; water intracristalinical fixated oxidrilic on a lamellae. The hydrating reactions take place quickly enough and in several minutes the calcium anions passing in the solution leads to the increase of the value of the C/S rapport to 2, a fact that transforms C-S-H (alpha) in C-H-S(beta) much richer in calcium anions fixated to the lamellas’ surfaces and then in 2-3 hrs in C-S-H(gamma), when on the tetrahedron’s surfaces the small crystals fixes, then bigger and bigger of C-H.

Tricalcic aluminates

• Tricalcic aluminates hydrates with very big speed producing diverse forms of C-A-H (aluminates of hydrated calcium), which crystallises in the shape of hexagonal plates (C4AH14 ), which fixes more C-H and very much water and passes through a cubical form, more stable –C3AH6 . But as in the solution we have also calcium sulphate ( added to the clincher grounding), appear quickly in accicular structure, hexagonal in section the etringit, a sulfoaluminate of hydrated calcium, at the beginning in a trisulfo aluminates C6 AS3 H32 shape, and after the exhaustion of the gypsum in the form of monosulfoaluminate C4ASH12 , and hydroaluminate C4AH14, in hexagonal form, which induce the growing bind [1,2,3]. Increasing the concentration of C and S, besides the acicular etringit, monosulfoaluminate of calcium hydrated is modified also in the shape of hexagonal crystals and it forms solid solution with the hydroaluminate. The volume of the solid etringit increases in time through fixation of water determined by more mechanisms such as osmotic pressure at the hydration's interface, crystallization pressure etc

The structuringIn a couple of ours interval since preparation, the cement paste is placed in the destined space in the well and

remain unmoving. In the next 3 – 4 ours it is taking place what we call the structuring which needs as more

time as bigger water – cement factor is.

The structuring is a complex physical-chemical phenomena through which appearing a putting

-together of the free solid particles or weak bond particles, in the intimacy of the larger cadre

of the hydration products and in them expansion, which produce the apparition and growing

of the number of contacts betwee The Hardening n them. During this process, in the mass of the paste is formed through the contacts between the solid products, a structure with an increasing mechanical consistency, structure which adheres also to the space's borders in which the paste is placed.

In the beginning of the process this structure is weak, so it can be destroy by

moving and it needs repaus to be restarts again. When 20-30 percents of cement mass is captured in these

hydration products, slurry becomes stiffness [5,6]. If before the formation of this structure, the paste is

exercising on the space's borders a pressure that is proportional with its density, so it is in a hydrostatic state,

after the formation of these structures, the pressure on the same limits is exercised only by the free water

with a certain electrolyte concentration-which is in the paste's mass and which pressure, is of course much

smaller than that exercised by the paste in the initial period of placement. This modification of the value of the

pressure exercised by the paste has major consequences in achieving the goal of the cementation

operation/procedure.

The Hardening

• In the next 2 – 3 ours, depending on the nature and proportion of the components and also of the environment's conditions, the paste is transformed in stone. This transformation takes place gradually, through continual increase of the volume of the solid products of the hydration reactions. These ones has a proper and big porosity – nearly 0.3, formed by micropores with dimensions less than 2.5 nm. These products are forming and inside of unhydrationed granules clogging up capillary pore system which has dimensions up to 10000 nm, and interrupting most of part the connections of this system [4]. The growing of this hydration products leads to the apparition of the more frequent interactions on the bigger and bigger surfaces. The resistance to compression is at the beginning much bigger that that at the traction, because it is a result of the frictions that takes place in the contact zones. The contacts/interactions are taking place through films and menisci of liquids, but also solid-solid of the same or different mineralogical nature. Other forces that contribute to increase the stone's resistance are in ascendant order of size [5]:

• -the van der Vaals forces, which are manifested at the contact through liquids and are generated by the electrical charges of the anions from the solution;

• -oxidrilic forces ( hydrogen bridges);

• -valence forces.

As the number of interaction increases, weight of these forces becomes bigger.

Volume change

• Absolute volume is the volume of the solid matrix of the paste which hydrates or of the stone.

The modifications of the absolute volume which are taking place during hydration as well as after the stone formed, following the hydration physic-chemical reactions which take place in the mass of the paste and of the stone.

• Apparent volume is the volume occupied in a vessel by a paste or by the formed cement stone.

The modifications of the apparent volume are taking place generally up to formation of the stone and in a smaller proportion, in large periods of time, after the stone is form

autogenous contractions

In drilling activity we pay attention to apparent volume. The weight of this volume changes are done in its reduction direction - are contractions and are named autogenous contractions - depending only by the reactions from the system.

The term autogenous is meaning, from a theoretical point of view, that the paste is isolated from the surrounding environment, so doesn’t exchange substance or heat with this.

Fig. 2.The scheme of autogenous contraction

FLUIDS (GAS) MIGRATION

presence of formation fluids;

- historic;- LWD & MPD;

fluid allowing entry into the wellbore;

- well pressure < formation pressure

flow path in the wellbore accessing a lower pressure zone- failure hydraulic bonds (micro annulus), channels

through cement or undisplaced mud

Presence of formation fluids

Evaluation of gas-flow severity (after Halliburton):

GFP = pR,max/pOB

Where: GFP is the gas flow potential factor;

PRmax – is maximum pressure reduction due hydration reactions of the cement;

POB – is the initial overbalance pressure for the gas zone

For GFP value between: 0 – 3 Low potential;

: 3 – 8 Moderate potential;

more than: 8 High potential

Pressure reduction in structured cement

RESULTS OF THE PRIMARY CEMENTATION

Gaz migration through mud channels

STR

Gas migration through cement sheath

Controlling migration phenomena

• Slurry and cement stone quality;Alter cement properties: thixotropic, gas generation, foam stabilizing surfactants

• Perfect mud displacement;

• Keep overbalance; Back pressure at surface; two stages of different set time slurry; slurry behavier modifications

• Flush formation gas into formation prior to cementing;• Flexible cement (lower Young`s modulus).

Mineral adding

This mineral additions are solid materials , fine milled , which has a chemical reaction with the hydrated cement, creating a modified microstructure of the paste. So we can distinguish materials with puzzolanic properties , materials with latency hydraulic properties and mixtures.

• a.- materials with puzzolanic properties, are those siliceous materials, that under the form of finely divided dust and in the presence of the water, reacts with the calcium hydroxide and forms cement components. The puzzolans can be of natural or industrial origin. The natural ones are the volcanic ashes and the diatomeic grounds. As industrial ones we have the flying ashes – inorganic waste from the coal burnings, silica fume (silicate residue) –rezulted from the gase formed in the making of the sillicium

• b.- latent hydraulic materials, react directly with the water and forms cement component. In this category we can find the slags furnace.

Chemical properties of Power PozzTMHRM , MetaverTM R, MetaverTM W and Microsit® M10.[5-8]

No. PozzolanName

SiO2 Al2O3 Fe2O3 TiO2 SO3 CaO MgO Na2O K2O MnO Lost at ignition

1. Power PozzTM

HRM52-54 42-44 <1-1.4 <3.0 <0.1 <0.1 <0.1 <0.05 <0.4 <1.0

2. MetaverTM W 54-56 36-38 <1.9 <1.0 <0.8 <0.9 <0.6 <0.4 <2.8 <2.5

3. MetaverTM R 67-69 25-27 <2.5 <1.5 <0.8 <0.1 <0.1 <0.2 <0.1 <1.5

4. Microsit ® M10 52 26 6 5 3,5

SAMPLE CODE

COMPOSITION OF THE PASTE

VARIATION OF VOLUME - CHEMICAL SHRINKAGECs%

3h 6h 9h 1zi 2 days 3 days 7 days 14days 28days 90 days

C CEMENT SIMPLU +0,05 +0,10 -0,2 -1,91 -2,57 -3,07 -3,72 -3,72 -3,82 -4,82

PP1 C+POWER POZZ 10% 0 -0,20 -0,41 -2,26 -2,58 -3.09 -4,23 -4,23 -4,43 -5,36PP2 C+POWER POZZ 15% 0 -0,45 -0,75 -2,20 -3,10 -3,45 -4,30 -4,30 -4,60 -5,35PP3 C+POWER POZZ 20% 0 -0,59 -0,92 -2,16 -3,07 -3,42 -4,46 -4,52 -4,85 -5,45

MR4 C+METAVER R 10% +0,20 +0,05 -0,10 -1,40 -3,00 -3,30 -4,40 -4,40 -4,70 -5,30MR5 C+METAVER R 15% +0,25 +0,15 -0.10 -1,20 -2,20 -2,80 -3,60 -4,18 -4,47 -5,40MR6 C+METAVER R 20% +0,20 +0,05 -0,14 -1,30 -2,33 -2,54 -3,50 -4,35 -4,50 -5,50

MW7 C+METAVER W 10% +0,20 +0,12 -0,10 -1,10 -2,52 -3.13 -3,94 -4,24 -4,44 -4,55MW8 C+METAVER W 15% 0 +0,10 -0,10 -1,60 -1,70 -2,80 -3,60 -4,00 -4,30 -4,50MW9 C+METAVER W 20% 0 0 -0,20 -1,40 -2,50 -2,85 -3,60 -4,00 -4,20 -4,40

MI 10 C+MICROSIT 10 10% +0,28 +0,40 +0,4 -1,40 -2,10 -2,60 -3,20 -3,40 -3,70 -4,70MI 11 C+MICROSIT 10 15% +0,18 +0,30 +0,31 -1,33 -2,24 -2,50 -3,27 -3,32 -3,78 -4,90MI 12 C+MICROSIT 10 20% +0,15 +0,21 +0,52 -1,35 -1,48 -2,45 -3,07 -3,44 -3,85 -5,42

. Chemical shrinkage of the pastes (%)

Perfect mud displacement

Centring casing;

Turbulent flowing;

Mud rheology;

Flowing time;

Pumped slurry volume

Effect of centralizers

Mud removing by casing rotation

Turbulent flow at low rheology

CEMENT SHEATH INTEGRITY

loging;

leak off test;

special cementing (eventual)

LEAK OFF TEST CHART

Schematic pressure chart for squeeze job using hesitation technique

Lost pressure in skin zone

Converting `slurry-stone` in three stages

Converting `slurry-stone` in three stages

Pressure reduction in low stage cement

The initial and final set time

SAMPLE CODE

COMPOSITION OF THE PASTE Initial set time Final set time

C SIMPLE CEMENT 5h15” 6h50”

PP1 C+POWER POZZ 10% 4h55” 6h25”PP2 C+POWER POZZ 15% 4h30” 6h15”PP3 C+POWER POZZ 20% 4h 5h

MR4 C+METAVER R 10% 5h30” 6h45”MR5 C+METAVER R 15% 5h45” 7hMR6 C+METAVER R 20% 6h 7h15”

MW7 C+METAVER W 10% 3h25” 5h40”MW8 C+METAVER W 15% 3h45” 6h45”MW9 C+METAVER W 20% 5h 7h15”

MICR10 C+MICROSIT 10 10% 4h45” 9h35”MICR11 C+MICROSIT 10 15% 7h30” 10h45”MICR12 C+MICROSIT 10 20% 12h45” 15h15”

The initial and final set time of the pastes