Tuesday, August 18, 2015

Invasion Profile: Transition Zone



This post is an elaboration of the "Mud Invasion Profile" slide which I share in my previous post [CLICK HERE]. This explanation will fit the best if you want to present the "Mud Invasion Profile" slide at page 5 until 6.





Please download the FREE slide in my previous post [CLICK HERE] to get the whole analysis, colorful demonstration, and easier understanding.



Introduction

When we see a wellbore design, the hole diameter of the borehole is translated as the outside diameter of the drill bit.
Unfortunately, the borehole diameter can be larger or smaller than the diameter of the drill bit and it is quite tricky to be measured. It might changes because of wash-out and/or collapse of the shale, poor cementation in porous rocks, or the thick mud cake on porous and permeable formations. The solution for real hole size diameter is by using the caliper log.


Caliper log (source)


While drilling, there’s always drilling mud inside the borehole, except special case like underbalance drilling or the worst of all, kick and blow out. Drilling mud helps move cutting of boreholes, lubricate and cool the drill bit, drill and maintain the excess pressure on the formation pressure, keep the wellbore stability, keep the drill cutting sustain when the drilling stops, and in mud motor, it can move the drill bit.

Mud density is kept high so that the hydrostatic pressure of the mud column is always greater than the formation pressure. This pressure difference pushes some mud seeping into the formation. But when the mud seep into the formation, solid particles stuck on the side of the hole and form a mud cake. They block the pores so no further seepage can go into the formation. The failure of mud cake to block more seepage of mud fluid will lead into mud loss, which will start well kick if not treated properly.

(mud cake formed when mud flow through filter paper. mud cake will prevent further seepage)

The mud fluid into the formation called mud filtrate (mud filtrate).  Small seepage of mud fluid will separate the formation condition into 2 zones. A zone infiltrated by mud filtrate is called invaded zone. Invaded zone consists of Transition/Anulus Zone and Flushed Zone. The zone that is not contaminated by mud filtrate is called uninvaded zone. It is only saturated by formation water, oil, or gas. Water saturation in this zone is very important, because it is used to determine the hydrocarbon saturation in the reservoir.

Invaded Zone Profile

Flushed zone is only a few inches from the borehole, this zone is usually cleared of formation water. All moveable formation fluid has been replaced by drilling mud. If there is oil, the invasion of mud filtrate can be determined from the difference between the water saturation in this zone versus the water saturation in uninvaded zone. Usually about 70-95% of oil moved. 

Transition or Annulus Zone appears when formation fluid and mud filtrate mixed. Transition zone occurs between the flushed zone and uninvaded zone.
Ideally, there are three types of fluid invasion distribution in the borehole.

The shape of transition Zone

We will see how the distribution of seepage in the invaded and uninvaded zone and its relationship with the relative resistivity.

 Step Profile, mud filtrate is distributed as a cylinder around the borehole. In the outer diameter of flushed zone, we can see a sudden contact with uninvaded zone, which is mud filtrate free. Uninvaded zone occupied by formation water or hydrocarbon. In this example uninvaded zone filled with 100% water (no hydrocarbon) so the resistivity reading is low. Thus, we see that there’s no transition zone, since mud filtrate doesn’t go to uninvaded zone partially.

(from George Asquith & Charles Gibson)

  
Transition Profile, This is the most realistic models. Distribution is still a cylinder, but the invasion of mud filtrate reduced gradually (gradation) in transition zone, then connect with uninvaded zone at the outer part. At Flushed Zone, pores filled mud filtrate, and the resistivity measurement will be high. On the Transition Zone pores filled with mud filtrate, formation water and, if any, residual hydrocarbons. At Uninvaded Zone, water filled pore formation, and if there is, hydrocarbon (in this diagram hydrocarbons does not appear since the resistivity measurement is low at uninvaded zone).
(from George Asquith & Charles Gibson)

Annulus Profile, describes the temporary fluid distribution when the logging operation is stopped for a moment (not recorded in the log). Annular profile describes the fluid presence between invaded and uninvaded zone. It is a sign of the hydrocarbons presence. This profile can only be detected by induction log (ILD or ILM) as soon as the well is drilled and show high resistivity measurement. When the mud filtrate seep into the zone, formation water is pushed out, then the pushed formation water form a ring (annular ring) on ​​the invaded zone boundary, this profile can only occur at the hydrocarbon bearing zone. At the Flushed Zone, pores are filled by mud filtrate and residual hydrocarbons, so the value of resistivity is high. On Transition Zone, pores are filled with mixture of mud filtrate, formation water, and residual hydrocarbons. Outside those zone is the Annulus Zone, where the pores are filled with formation water and hydrocarbons. At the time of the annulus profile appear, the resistivity will suddenly decreased on the outer boundary of invaded zone, due to the high concentration of formation water. Formation water seepage is pushed out by mud filtrate into the annulus zone. This leads to the temporary absence of hydrocarbons in annulus zone. Basically, most of the hydrocarbon bearing rock is water wet, and the relative permeability of the oil is higher than the water. That’s why it is easier to remove the oil in annulus zone because the mud filtrate pushes the formation water to annulus zone. It is temporary because at the later time the hydrocarbon will push back the formation water to create balance.  The formation water resistivity measured in annulus profile zone will be higher than in real condition because it is affected by high resistivity hydrocarbon.
(from George Asquith & Charles Gibson)



Transition of Resistivity Profile

Whether the resistivity shown in invaded zone is higher or lower than the uninvaded zone will depend on the drilling mud being used in the wellbore and the formation fluid detected in the penetrated zone.

When using fresh water drilling muds, in water bearing zone, the mud filtrate resistivity will be lower than that of formation water. That’s why the transition zone will have lower resistivity than the flushed one, lower and lower until the uninvaded zone.

 Using the salt water drilling mud, the resistivity of mud and formation water in this zone will not have significant difference. Invaded zone, flushed zone and uninvaded zone will have low resistivity result.

resistivity transition through water bearing zone; upper: using fresh water mud. Lower: Using salt water mud (from George Asquith & Charles Gibson)

The drilling somehow will penetrate the hydrocarbon bearing zone (water saturation less than 60%). Using fresh water mud, the resistivity of flushed zone is high, since fresh water have higher resistivity than the formation water. In transition zone, typically the resistivity is still high, since it contains mud filtrate and hydrocarbon, even though some portion of formation water would be there. In uninvaded zone, there’s no portion of mud filtrate and more portion of formation zone. Thus, the resistivity is lower.


If we use the salt water drilling mud, which is low in resistivity, we can see that in flushed zone the resistivity is low, then the transition zone have higher resistivity since hydrocarbon has appeared in this zone, and the uninvaded zone will have the highest resistivity since the hydrocarbon saturation in uninvaded zone is the original hydrocarbon saturation.

resistivity transition through hydrocarbon bearing zone; upper: using fresh water mud. Lower: Using salt water mud (from George Asquith & Charles Gibson)

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