Sophisticated SLURRY SYSTEMS

Shale gas development largely uses oil-based mud for horizontal well drilling, which provides hole stability, a rapid rate of penetration and reusability. There is also an emphasis on large-scale fracturing technology in order to

maximise production capacity and develop greater economic value. These factors

VICTOR HUANG AND JOHN ZHU, VERTECHS, DISCUSS THE TECHNOLOGIES BEHIND SUCCESSFUL CEMENTING JOBS IN SHALE GAS RESERVOIRS.

Sophisticated SLURRY SYSTEMS

| Oilfield Technology Reprinted from November 2016

mean that it is essential to provide sophisticated operational engineering and products to execute successful cementing jobs in the shale gas reservoir. Complex factors such as the effective displacement of oil-based mud, cement slurry strength development issues over high temperatures and large differential temperature environments make cement slurry a critical design consideration. Additionally, the elastoplastic mechanical properties necessary for a properly performing cement sheath make proven technology and knowledge crucial to success. When the formations are capable of both pressure and losses in the same section, this situation ‘creates’ a narrow safety margin or ‘window’ for fluid density. This narrow margin increases the challenge for successful cementing operations. Control of the slurry, equivalent circulating density (ECD) during the cement pumping operation is vital. If ECD is too low, the resulting pressure instability can allow formation fluids (connate or lost) to return to the wellbore and can contribute to gas channelling while WOC. If the ECD is too high, reduced returns due to losses can compromise the cement integrity. Both extremes in ECD values contribute to poor results or failure. Vertechs Multi-FIT cement slurry is a customised solution aimed at providing stable cement performance and results for zones known for: elevated temperature; abnormal pressure; large differential temperature; shale gas and horizontal well completions; especially the cementing complexities to consider when following a drilling operation using oil-based mud.

Characteristics of the slurry system Ì Suitable for large differential temperature well conditions

(Up to 120˚C variance). Ì All additives are stable under high temperatures (up to

220˚C) Ì Applicable to density range from 1.10 SG to 2.90 SG.

Ì Compatible with OBM/WBM. Ì Designed to prevent losses and provide improved sealing

properties. Ì Enhanced elastoplastic properties for mitigating the annular

pressure issue from large-scale fracturing or other pressure sources.

Cement additives Ì FlexPlus is an elastoplastic additive developed to improve

the comprehensive mechanical properties of cement, such as: rupture strength; impact toughness; decreased cement elasticity modulus, improved stress and strain capacity of cement. These features prevent external force damage to cement which may cause micro-gap or micro-annulus that will allow communication and compromise the cement bond or integrity. All these benefits improve the cementing quality and extend the life of the well.

Ì VXR: A broad-spectrum type retarder with the capacity to work over a middle to high temperature range. This unique feature provides obvious advantages in long section cementing for deep wells, particularly with a high temperature (up to 220˚C) or large differential temperature (up to 120˚C variance). VXR provides tremendous stability in premixed form up to 30 days – held in reserve (stored) until mixed into the slurry.

Ì VFL: VFL fluid loss agent is a cement additive that resists salinity up to a saturated concentration with temperature tolerance up to 200˚C. It is able to control the fluid loss of cement at high temperature and minimise the micro-annulus, to reduce gas channelling.

Ì Fiblok: A specialty sized fibre with a monofilament structure. Its high dispersion feature ensures it has no adverse effect in mud/cement pump, drilling tools, BOP, bit nozzles. Fiblok is compatible with WBM, OBM and does not elevate drilling fluid rheology; is insoluble in drilling fluids; can be removed from

shale shaker along with cuttings. Low effective dosage required (less than 1 kg/1 m3 drilling fluids).

ApplicationsThe Vertechs Multi-FIT slurry design has been used successfully in multiple sections on multiple wells in a major operator’s shale gas project. The company has cemented numerous, challenging, deep (up to 6000 m) horizontal wells over three years’ service from 2013 to 2016.

Figure 1. Vertechs Fiblok – right image shows cuttings.

Table 1. Applications of Multi-FIT cement slurry system.

Top/bottom Experiment pressure MPa

Slurry weightSG

OBM weightSG Cementing result Annular

pressure

36/136 83 2.00 - 2.30 1.74 - 2.14 Excellent No

40/129 109 2.04 - 2.40 1.84 - 2.00 Excellent No

34/150 89 2.04 - 2.30 1.84 - 1.94 Excellent No

Max Temp. 136˚C, highest pressure is 103 MPa, range of temperature up to 100˚C, maximum slurry density 2.40 SG. Multi-FIT successfully mitigated chronic problems such as poor cement bond, annular communication (gas & pressure) and continues to be the choice of this operator’s team.

Reprinted from November 2016 Oilfield Technology |

Example case historyChallenges included: high density environment, long open hole section, wide range of temperature, single stage, horizontal angle, losses and kicks while drilling.

Before the cementing operation was planned, drilling the bottom hole section presented several difficulties. As the drilling fluid density climbed to 2.20 SG seepage losses ensued. With a reduced density at 2.15 SG the well kicks gave a gas flare up to 10 m, indicating that the safe pressure window is narrow.

During the first casing run the string could not reach the desired depth, so the casing was pulled and the centralisation

equipment reconfigured with new technology and new types of centralisers. The company provided one-piece spring centralisers and low friction coefficient rigid centralisers for combined use. Vertechs centraliser design software was used to simulate and optimise the centraliser placement. The casing was re-run and landed in place ready for cementing operations. Before cementing, Fiblok was pumped to sweep the hole clean and ensure no debris remained.

The company’s retarder can be pre-mixed up to 30 days in advance, which means substantial savings for operators, by thickening time curve on the same premixed fluid. Multi-FIT lead slurry ultra-sonic strength development curve under low temperature shown in Figure 2.

With the development of drilling technology, and the requirement for the cement slurry; HT, long cementing interval, high temperature difference, cement sheath compressive strength and thickening time requirement challenges the retarder to deliver results. The key design requirement was that the lead slurry compressive strength needed to build up within 72 hrs at 35˚C.

Planning for Multi-FIT pumping operations indicated that a 1.17 SG NaCl brine be chosen as the spacer and a pumping pressure up to 60 MPa. Due to the narrow density ‘window’,

pressure was carefully controlled during the pumping and adjusted constantly by varying the pump rate to control ECD accordingly.

The pumping was adjusted constantly by varying the pump rate to control ECD accordingly.

Multi fit key elements and functional mechanism

Primary bridging As the Multi-FIT cement slurry passes through a porous or permeable formation, it creates a bridging effect on the surface and into the pore throat as the basic foundation of the bridge.

Filling and embedding effect After bridging, there is a basic skeleton on the surface of the porous formation, the thin particles in the slurry material gradually fill and embed into the tiny holes of the basic bridging skeleton. The differential pressure causes compaction, slowly forming a blocking partition to reduce or to eliminate losses, improve the loading ability of fluid column pressure (ECD/ESD).

Infiltration and brace effectAs the Multi-FIT cement slurry forms filter cake under differential pressure and is packed into loss zones, selected fibrous material is caught in the filter cake and has a strong bracing effect. This kind of filter cake containing additives to reinforce bridging capacity, combined with the supporting skeleton can be used to fill and embed chosen bridging material. This reduces or eliminates losses and enhances a plugging effect to improve the limit of hydrostatic or pump pressure that can be necessary for cement pumping operations.

As the pumping plan ECD curve shows, with a narrow pressure window it is important to ensure well control safety and also take losses into consideration to reduce loss volume during displacement.

Figure 2. Multi-FIT lead slurry ultra-sonic strength development curve under low temperature.

Table 2. Horizontal well in Southwest China shale gas field.

Depth (s) 5520 m MD/3360 m TVD

BHST 130 ˚C 130˚C

TOC: 300 m 300 m

TOC Temp: 40˚C

Maximum angle: 94˚

Cement stage (s): Single

Drilling fluid OBM - 2.5 SG (86:14 OWR)

Cement slurry density 2.20 SG

Figure 3. Bottom hole ECD curve (no losses observed after slurry displaced into annulus).

VertechsOil&GasTechnologyCo.,Ltd.

Formoreinformation,pleaseemailtoengineering@vertechs.comVERTECHS®—YourOne-SourceOil&GasSolutions

Multi-Fitleadslurryultra-sonicstrengthdevelopmentcurveunderlowtemperatureshowninPicture-2.

Withthedevelopmentofdrillingtechnology,andtherequirementforthecementslurry;HT,longcementinginterval,hightemp.difference,cementsheathcompressivestrengthandthickeningtimerequirementchallengestheretardertodeliverresults.Thekeydesignrequirementisthattheleadslurrycompressivestrengthneedstobuildupwithin72hrsat35°CPlanningforMultiFitpumpingoperationsindicatedthata1.17SGNaClbrinebechosenasthespacerandapumpingpressureupto60MPa.Duetothenarrowdensity ‘window’,pressure iscarefullycontrolledduringthepumpingandadjustedconstantlybyvaryingthepumpratetocontrolECDaccordingly.thepumpingandadjustedconstantlybyvaryingthepumpratetocontrolECDaccordingly.MULTI-FITKEYELEMENTS&FUNCTIONALMECHANISM

1. PrimaryBridging AstheMulti-FITcementslurrypassesthroughaporousorpermeableformation,itcreatesabridgingeffectonthesurfaceandintotheporethroatasthebasicfoundationofthebridge.2. Fillingandembeddingeffect

Afterbridging，thereisabasicskeletononthesurfaceoftheporousformation,thethinparticlesin

the slurrymaterial gradually fill andembed into the tinyholesof thebasic bridging skeleton. Thedifferentialpressurecausescompaction,slowlyformingablockingpartitiontoreduceortoeliminatelosses,improvetheloadingabilityoffluidcolumnpressure(ECD/ESD).