Difference Between Leak off test, Formation integrity test & Formation breakdown test

While Leak off test, Formation integrity test & Formation breakdown test all have the same procedure and aim, the only difference is the point to which the casing shoe will be pressured.

LOT,FIT & FBT

Formation integrity test (FIT) is conducted when we already know how much maximum pressure next open hole section could experience.for e.g if Maximum pressure open hole section could experience is 2600 psi we will just pressure the casing shoe slightly higher than 2600 psi pressure for safety margin. If at this pressure formation is intact we will have have a straight line plot between Surface pressure and Cumulative volume i.e in the above plot it will lie between point ‘o’ and ‘a’ and the test will be called Formation integrity test.

if the test is conducted to the point when the drilling fluid starts to leak into the formation i.e between point ‘a’ and ‘b’ it is Leak-off test (LOT). Between point ‘a’ and ‘b’ only micro-fractures have developed into the formation which are very small in size and are recoverable, if the pressure is reduced will again close.

‘b’ is the point at which actual fracturing takes place and these fractures are permanent in nature.If the test is conducted beyond ‘b’ it will be termed as Formation breakdown test (FBT). Remember it is not recommended to go beyond point ‘b’ as by doing so we will be inducing permanent fractures into the formation which can cause lost circulation while drilling ahead.

Leak off test procedure and calculation

Leak off test is conducted to measure the pressure at which the drilling fluid just start to leak into the formation or the formation just start to fracture.Alternatively leak-off pressure is the maximum pressure which the open hole section can bear without fracturing and from this maximum pressure we can calculate the maximum mud weight which could be used to drill this open hole section.It is conducted in the new formation after the casing has been set.
  • Leak-off test
  • Formation integrity test
  • Formation breakdown test/Fracture test
All the above pressure tests are the same and are conducted to measure the strength of the weakest part of the next hole section to be drilled. For these pressure tests it is assumed that the shallowest depth of the open hole section has the least strength because as we move deeper compaction increases (due to the weight of overlying formation) and so does the strength of the formation.if we make sure that during any operation(drilling, tripping and casing) in the open hole pressure does not exceed the leak-off pressure there will no chance of fracturing the open hole.

Procedure:-
  1. After the casing and cementing , 3 to 5m of next hole section is drilled.
  2. Drill string is pulled into the casing shoe.
  3. Close the BOP.
  4. Equal volumes of Drilling mud is pumped in intervals(Genrally 1-2 bbl) from the cementing unit.After pumping each volume, pressure will increase as the well is closed.
  5. Surface pressure and cumlative volume is plotted on a graph.
  6. After the test is finished, pressure is bleed-off from the surface and the BOP is open.
LOT graph
Initially the graph is straight line showing surface pressure is directly proportional to the cumulative volume being pumped.After point “a” graph deviates from straight line relationship but still the pressure will increase with volume.This deviation shows that drilling fluid has started to leak into the formation.At point “b” pressure decreases drastically which indicates formation has been fractured.
Remember normally under static condition( no circulation) pressure at any point inside the well-bore will be the hydro-static pressure due to mud weight but during LOT well bore pressure will be:-
Pressure at any point= surface pressure + hydro-static pressure
Surface pressure is due to pumping drilling fluid into the well while the BOP is shut and this surface pressure is plotted against cumulative volume pumped.but the actual well-bore pressure profile looks like:-

Hydrostatic pressure profile

Calculation:-
Surface pressure when leak off started=1000 psi
TVD=5000 ft
Mud weight= 10 ppg
Leak off pressure or Maximum allowable pressure at shoe= surface pressure + Hydro-static pressure due to mud weight
                                                      = 1000 +0.052*TVD(ft)*MW(ppg)                                                       =1000+0.052*5000*10                                                       =3600 psi For this example at 3600 psi formation below the casing shoe will start to fracture so we will plan further operations such that pressure at shoe never exceeds this value. From this Maximum allowable pressure, maximum mud weight which could used to drill the next section can be calculated.
Maximum Mud weight (psi/ft)=Maximum allowable pressure at shoe/(0.052*TVD)                                                           =3600/(0.052*5000)=13.84 ppg
If Mud weight exceeds 13.84 ppg while drilling the next hole section formation will fracture so mud weight should be less than 13.84 ppg.If for any reason mud weight has to be increased we will first case and cement  the open hole than increase the mud weight.

Note:- Generally Cement unit pumps are used for Leak-off test as they are more accurate than Rig pumps.

Lost Circulation

Lost circulation is a drilling problem where the drilling fluid is partially or completely lost in to the formation.During Loss circulation, Pit volume and mud flow out will decrease. Ideally while we are drilling, there should be very less decrease in drilling fluid volume which is required to fill the extra hole drilled.But in practice more volume of drilling fluid is lost while drilling which is lost into the formation. According to severity losses can be classified as :- 1. Seepage 2. Partial 3.Severe 4. Complete Loss.
 
In overbalance condition, hydrostatic pressure of drilling mud is greater than formation pressure which pushes the liquid part of drilling fluid into the formation while the solids are deposited on the borehole wall and forms the filter cake which prevents further losses.The liquid part which is lost to the formation is called filtrate and the Losses while the filter cake is depositing across the borehole wall are termed as Spurt Losses.

Drilling fluid

Equivalent circulating density-ECD

Equivalent circulating density is the density of drilling fluid during circulation.Actually mud density remains the same under static or circulating condition but the pressure under both the condition differs.When the drilling mud is under static condition(no circulation),Pressure at any point is only due to drilling mud weight and is given by:-

Pressure under static Condition = 0.052* Mud weight(in ppg)* TVD(in feet)   (equation-1)

But during circulation, extra pressure is applied by the mud pumps to circulate the drilling fluid. This pump pressure is equal to the total pressure loss in the circulating system.That’s why during circulation, pressure at any point in the wellbore will be due to mud weight plus the pressure loss in the system.

Circulating pressure = Pressure under static Condition + pressure loss in the system
Drilling mud pump pressure loss

ECD is always calculated for annulus as it has no importance inside the drill string that's why we can replace pressure loss in the system with annular pressure loss.

Circulating pressure in the annulus = Pressure under static Condition+ Annular pressure loss (equation-2)

ECD can also be defined as the density equivalent of circulating pressure in the annulus.

Circulating pressure in the annulus= 0.052* ECD* TVD                                     (equation-3) Putting equation-3 in equation-2 we get:-
0.052* TVD* ECD= Pressure under static Condition + Annular pressure loss

ECD=(Pressure under static condition + Annular pressure loss) / (0.052* TVD)

ECD=Mud weight + Annular pressure loss/(0.052*TVD)

Importance of ECD:-

For safe drilling it is must to keep the ECD lower than the formation fracture gradient otherwise the formation will fracture that's why ECD is continuously monitored while drilling.

ECD < Fracture pressure gradient                    to avoid fracturing the formation
Mud weight >= Pore pressure                           to avoid kick

Note:-During circulation ECD will always be greater than or equal to Mud weight.

Drilling Fluid-Primary well control

Following are the main functions of drilling mud:-

  • To remove the drilled cutting out of hole
  • To control the formation pressure for safe drilling-Primary well control
  • To cool and lubricate the bit and drill string
  • Formation of good mud cake to avoid losses
In this post we are going to discuss primary well control.
Generally we drill under overbalance condition. Overbalance means borehole pressure is higher than formation. Typically as formation pressure increases, drilling fluid density is increased with barite to balance the formation pressure. This keeps formation fluids from entering into the wellbore and prevents pressured formation fluids from causing a blowout.
Drilling fluid

Drilling fluid properties-Part 3-Yield point and Gel stength

Gel strength is the measure of the ability of fluid to develop gel structure and retain it when the mud is at rest. Due to gel strength mud behaves as semi solid when at rest.When the mud is at rest it develops a semi solid structure due to the attractive forces between particles of mud resulting from the presence of positive and negative charges on the surface of these particles.

Yield point is the point at which semi solid structure of mud breaks and the mud is about to flow.It is minimum stress or pressure applied at which mud will start to flow.

Importance of Gel strength and yield point:-

Both yield point and gel strength affects the cutting carrying capacity of mud.Gel strength developed in mud under static condition holds the drilled cuttings in suspension. Higher the gel strength higher will be the yield point and higher pump pressure will be required to break the circulation. This high pump pressure is undesirable as it may fracture the weaker formations.

Bingham plastic model

Field measurement of Yield point and Gel strength:-

In field, yield point and gel strength both are measured from Rotary viscometer along with Plastic viscosity.
 what type of fluids does drilling fluid belong to, gel strength vs yield point, what is plastic viscosity and yield point?,

Yield Point=Viscometer Reading at 300 rpm-Plastic viscosity

For Gel Strength measurement, first the mud is sheared(agitated) at high rpm(600) for about 15 seconds and then allowed to rest for 10 seconds then again it is sheared at 3 rpm and the torque reading is noted as Gel strength after 10 seconds.If the Gel strength after 10 mins or 30 mins are required same procedure is repeated just the time for which the mud was at rest will change. Generally gel strength after 10 seconds or 10 min are reported.

Field unit of Gel strength and yield point is lb/100ft2

Drilling fluid properties-Part 2- Mud Viscosity

Fluid viscosity is the measure of friction between the layers of the fluid.Due to this friction fluid resists flow.Viscosity also tells about the thickness of the fluid.More viscosity means more thick fluid and more pressure will be needed to flow it.Viscosity depends on velocity and pattern of flow that's why absolute viscosity is difficult to measure and only changes in mud viscosity are important.

Main function of viscosity in drilling is to lift drilled cuttings.Annular Pressure losses also depends on viscosity.Higher the viscosity of drilling fluid higher will be the cutting carrying capacity(desired) but annular pressure losses and swab and surge pressure will also be higher(not desired).That's why a compromise is made and an optimum value of viscosity is chosen.

Also Read:-Drilling Fluid properties-Part 1-Mud weight , Part 3-Yield point and Gel stength

Field Measurement of Viscosity:-

In field, Viscosity is measured in two ways:-
  • Marsh funnel gives Marsh funnel viscosity
  • Rotary viscometer gives Plastic viscosity
Drilling mud viscosity


Marsh funnel Viscosity:- Marsh funnel works on the principle that thick fluid will flow slower than thin fluid. To measure viscosity first the marsh funnel is filled with drilling mud up-to a marked point (1500 ml).Then the time taken by the marsh funnel to discharge 946 ml (one quart) is noted and this value is taken as marsh funnel viscosity.Its unit is seconds/quart.

Changes in the value of marsh funnel viscosity tells that the mud quality is degrading and it needs treatment.This change in mud viscosity can be due to addition of drilled solids or influx from the formation. For fresh water marsh funnel viscosity is around 25-26 sec/quart.Though Marsh funnel viscosity don't have scientific support but it is the fastest way to check changes in mud quality.

Plastic Viscosity:- Plastic viscosity is measured by rotary viscometer which consists of two cylinders. Outer cylinder can rotate at various speeds while the inner cylinder is stationary and is connected to a dial through a spring. There is very less distance between the two cylinders that only very thin layer of fluid exists between them.When the outer cylinder is rotated it transmits torque to the layer of fluid between the cylinders which is transmitted to the inner cylinder which will deflect the dial and gives the reading of shear stress(torque) for that particular shear rate(RPM of outer cylinder)

            Plastic Viscosity (PV) = Reading at 600 rpm – Reading at 300 rpm
Its unit is centipoise(cp).

Drilling fluid viscosity
If the viscosity is to be increased viscosifiers such as bentonite,PHPA copolymers etc are added. If the viscosity is to be reduced water can be added.

Note:-Marsh funnel viscosity is measured regularly while drilling(every 30 min or 1 hour) both at suction pit(mud into well) and shale shaker (mud out) while plastic viscosity is measured by mud engineer once or twice per shift(or as per requirement)

Difference Between Leak off test, Formation integrity test & Formation breakdown test

While Leak off test, Formation integrity test & Formation breakdown test all have the same procedure and aim, the only difference is t...