Shell ResearchRijswijk

1992 Special Issue

Slimhole Drilling, Evaluation'and Completion

Advances in EP Research

All rights reserved

© Shell Internationale Petroleum Maatschappij B.V. 1992

The drilling of deep

exploration and development

wells with advanced slimhole

technology will require a

totally new type of drilling

fluid. On the one hand, the

fluid must not unduly resist

being pumped down a

slimhole well. Yet on the other

hand, it must have sufficient

density to keep the well under

control at all times and

sufficient viscosity to sweep

debris from the borehole -

even at high temperatures. A

family of high-density organic

brines of formate salts appear

to have these qualities and a

host of others that make the

brines remarkably suited for

the basis of such a drilling

fluid.

Advancesin Research

FORMATE-BRINE DRILLING FLUIDS

Compatiblewith oilfield

hardware

Minimalenvironmentalimpact

Cuttings

Figure 19Characteristics of an ideal slim hole drilling fluid.

20

P""~~~~~=============-""""-----------------------------------

The trouble withconventional drilling mudsAs wells are drilled deeper, theformation pressuresencountered by the drill bitgenerally increase. The drillingmud accordingly must bemade heavier and heavier tooffset the formation pressure.Ordinarily, drilling muds areweighted through the additionof dense solid particles such asbarite (barium sulphate). But asolids-weighted drilling mudcan suffer from high frictionalpressure losses when it ismade to circulate in theconfined spaces of deep slimholes (Figure 20). Such lossesmean that the hydraulic powertransmitted to the down holemud motors would be low;the ECD in contrast would behigh - as would the swab andsurge pressures arising fromthe raising and lowering of thedrillstring. The low hydraulicpower transmission reducesthe drill bit's rate ofpenetration, whereas the highECD and swab/surge pressurespose a significant threat toborehole stability and wellcontrol.

Solids-weighted muds are alsoconventionally treated in away that leads to thedischarge of whole mud andbarite along with the drillcuttings. Moreover, manyimportant qualities ofconventional drilling mud,such as viscosity and thecapability of stemming fluidloss into permeableformations, are degraded bythe high temperatures that arelikely to be encountered indeep wells.

The ideal slim hole drillingfluidAs depicted in Figure 19, adrilling fluid for a deepslimhole well should consist ofa low-viscosity, solids-free,dense brine containing aminimum of thermally stable

polymeric additives. In theinterests of cost efficiency andenvironmental protection thebrine system should becirculated in a closed cycle: allthe drilling fluid that returnsfrom the well should berecovered, regenerated andrecycled back into the well.The brine should be non-toxic,biodegradable and chemicallycompatible with oilfieldhardware and all drilling-fluidcomponents andcontaminants.

Does such an ideal slimholedrilling fluid exist? Very nearly.Shell researchers havediscovered that aqueoussolutions of the alkali metalsalts of formic acid appear tohave the right combination ofproperties.

Formate-brine systemsSodium and potassiumformate have roughly thesame density as table salt buta far higher solubility in water.When they are mixed in water,the resulting low-viscosity,slightly alkaline brine can havea density as high as 1.6 g/ml.Caesium formate, anothercompound of the same family,can more than double thedensity of the water to whichit is added. By adjusting theconcentration of the differentformate salts, one can meetnearly all drilling fluid weightrequirements without resortingto the addition of solidweighting agents.

To imbue the brines with someviscosity so that they canefficiently sweep thebottom hole clean of cuttings,xanthan gum can be added.This microbial polysaccharide isa widely used viscosifier forwater-based drilling fluids. It isalso one of the fewcommercially available water-soluble polymers that canmake a drilling fluid shear-thinning over a wide range of

Depth (m)0,-----,---,-.

1000

2000

3000

4000

5000-+--,----,---,---,------1o 100 200

Dynamic pressure (bar)

Figure20Calculated pressure-loss

distributions for

conventional chalk (red)

and formate drilling fluids(green) circulating at the

same rate in a slimholewell.

temperature, pH and salinity.Shear-thinning - a reversiblereduction in viscosity due toshear flow - is a favourableproperty for a drilling fluid,since it minimises thehydraulic pressure losses in thedrillstring and annulus.Xanthan gum is also known tohave a high elastic modulus,which means that it can keepcuttings in suspension underlow shear or even staticconditions.The drilling fluid must also becapable of sealing off zonesinto which it seeps, so that aslittle as possible of the fluid islost to the drilled formations.An exhaustive screening offluid-loss control polymers ledto the adoption of twocomponents that combined

effective high-temperatureperformance with minimaladded viscosity. The high-temperature performance ofthe fluid-loss control agentscould be further enhanced bythe addition of fine calciumcarbonate particles. Drillingfluids characterised by goodfluid-loss performance atbottom hole temperaturesreduce the incidence of stuckdrilling and loggingequipment, improve formationevaluation and minimiseformation impairment.

Thermal and rheologicalbehaviourTests have confirmed thatthese alkali metal salts arenon-toxic and biodegradable.Moreover, at their natural pH,the salt solutions show littletendency to corrode ferrous-based metals used in oilfieldtubulars. Possibly the mostoutstanding feature of theformate brines, however, isthat they reduce the rate ofdegradation of importantdrilling-fluid viscosifiers (suchas xanthan gum) and fluid-lossagents at temperatures above150°C.

Two experimental drillingfluids combining a formatebrine, the xanthan gumviscosifier and the polymericfluid-loss control agents (pluspH control agents) havealready been formulated tomeet the needs of a Shelloperating company planningto drill slimhole wells deeperthan 5,000 metres. On thebasis of their fieldperformance and detailedhydraulic calculations, evenbetter formulations may bedeveloped.

21 Advances in EPResearch