THE CORRELATION BETWEEN POROSITY, SATURATION, PERMEABILITY AND ACID SOLUTIBILITY ON CORE SAMPLES OF

LIMESTONE ROCKS IN THE KEMADANG AREAKorea National Oil Team

AbstractPetrophysics is the study of rock properties and their interactions with

fluids (gases, liquid hydrocarbons, and aqueous solutions). The geologic material forming a reservoir for the accumulation of hydrocarbons in the subsurface must contain a three-dimensional network of interconnected pores in order to store the fluids and allow for their movement within the reservoir. Thus the porosity of the reservoir rocks and their permeability are the most fundamental physical properties with respect to the storage and transmission of fluids

Rock core analysis in the laboratory is used to determine the petrophysical properties of rocks. Rock core analysis procedure basically consists of two parts, they are routine rock core analysis and special rock core analysis. Routine rock core analysis porosity in general is about porosity measurement, absolute permeability measurement and fluid saturation measurement. Whereas special rock core analysis can be grouped into two, they are the measurements in static condition and measurements in dynamic conditions. Measurements in static conditions including capillary pressure, electrical properties and the speed of sound propagation, grain density, wettability, rock compressibility, permeability and porosity function of pressure (net over burden) and petrographic studies. And the measurements on dynamic conditions include: the relative permeability, thermal recovery, residual gas, water flood evaluations, liquid permeability (completion evaluation, work-over and the injection fluid includes surfactant and polymer).

From the laboratory experiments that we did, there is the some results. The are measurements of porosity, permeability, saturation, acid solutibility of formation samples. Core that we used for research was limestone because limestone is a carbonate rock that will react with acid, and it is also porous rock according to the physical properties of reservoir rocks in the field. We took that core in the Wonosari area, because Wonosari is an area of limestone mountains.

Keywords : Porosity, Saturation, Permeability, Acidizing.

1. PREFACE1.1. Background Issues

Rock core analysis is the analysis steps after core sampel of formation in the subsurface, obtained in which the purpose of the rock core analysis is to determine directly the information of the physical properties of rocks to be carried out exploration. Core analysis is divided into two activities, The are Routine Core Anaysis and Special Core Analysis. Routine Core Analysis include the measurement of porosity, saturation and permeability.

Porosity measurements or samples of reservoir rocks is to know the size of the cavity in the rock by rock bulk volume and pore volume of the rock. Measurement of Fluid Saturation aim to determine the pore space of reservoir rocks containing fluid especially oil. Permeability measurements to determine the ability of a fluid reservoir rocks passed without damaging the structure of the constituent rocks. Determination of Levels of Sample Solution Formation in Acid Solution, determine the concentration of acid-soluble formations in solution, in order to obtain important information prior to stimulation.

1.2. Methodology The method used to

determine the porosity is Mercury Method Injection Pump, with materials used Hg or mercury as a guide. It is also used weighing method. But we

only use the weighing method. The method used for the measurement of saturation is a method of distillation and retort method. But this time only just introduced distillation method. To determine the permeability of a reservoir rock that is supported by a tool consisting of a liquid permeameter and gas permeameter. This tool can only identify a single fluid, the rest of this tool can not be read.

For samples soluble levels, we calculate the solubility as% by weight and material dissolved in 5N HCl. In experiments sieve analysis, we determined the type of screen or filter to be used to fix the sand problem.

2. THEORETICAL 2.1. Porosity

Discussed from the point of reservoir engineering, porosity divided into two, namely: Absolute porosity, defined as the ratio between the whole pore volume to the total volume of rock, or can be written:∅abs=Vp/Vbx100%........................ (1)∅abs = (Vb-Vg)/Vb x 100 % ..........(2)

Vp = volume of rock pores, cm3 Vb = the total rock volume, cm3 Vg = volume of grain, cm3

Effective porosity, defined as the ratio of the pore volume is related to the total volume of rock or written:

∅eff = (Vp related)/Vb x 100 % ....(3)

Therefore, the oil can only flow through interconnected pores. So that is important in the petroleum industry and that we measured in this

experiment is the effective porosity. In general, the amount of porosity ranging between 5-30%. Theoretically, the magnitude of the porosity of not more than 48%. On the field, we can get an estimate visually, which is semi-quantitative determination and is used as the following scale: 0% - 5% very bad porosity (can be ignored)5% - 10% bad porosity 10% - 15% sufficent porosity 15% - 20% well porosity > 25% excellent porosity2.2. Saturation

Reservoir fluid saturation is defined as the ratio between the volume of a particular fluid (water, oil, or gas) to total pore volume. Generally, the formation of the water content in the oil zone is called interstitial formation water or connate water. Water saturation is defined as:

Sw=water−filled pore volumetotal pore volume

……...(4)

Oil saturation is defined as:

So=oil−filled pore volumetotal pore volume

.......................(5)

Gas saturation is defined as:

Sg= gas−filled pore volumetotal pore volume

.............(6)

Sw + So + Sg = 1 ..................(7)

2.3. PermeabilityBasis for determining the

permeability of the rock is from trials conducted by Darcy. In the experiments, Henry Darcy using sandstone that is not compact by flowing water. The porous sandstone saturated with 100% fluid viscosity, cross-sectional area A, length L. Then, by applying pressure P1 entrance at one end, then the flow occurs at a rate of q, while the external pressure P2. In this experiment it can be shown that q is constant and will be equal to the price of the rock permeability is independent of the fluid, the pressure difference and the dimensions of the rock used.

Because Henry is considered a pioneer darcy permeability’s investigation then is Darcy permeability for the unit. Permeability itself is divided into two, namely: 1. Absolut Permeability is the permeability where fluid flow in porous media consists of only one phase, ex: only gas, oil, or water. 2. Effective Permeability is a permeability where fluid is flowing through a porous medium is more than one phase, ex: oil and water, oil and gas, water and gas, or all three. 3. Relative Permeability is the ratio between the effective permeability to absolute permeability.

Definition API to 1 Darcy is a porous medium having a permeability of 1 Darcy if single phase fluid with viscosity 1 cp, flowing with a speed of 1 cm / sec, through the cross-sectional area of 1 cm at a hydraulic gradient of 1 atm (76.00 mmHg) per cm and the liquid fills the medium. Mathematically it can be written as follows:

)P (PA Lμ x x Q K 2 1

................. (8)

Where in:K = permeability, Darcyq = flow rate, cc/secų = Viscosity, cpA = cross-sectional area, cm²L = length, cmP1–P2 = pressure difference, atm

In Darcy's law itself there are assumptions that are used, as follows: a) linear flow. b) in compressible fluid (can not breath / full because if crowded / full it does not cause the volume decreases. c) steady flow conditions (no change in the P and T). d) homogeneous and isotropic porous media. In units of the field K is expressed in miliDarcy. In the field estimate can also be made semi-cumulative giving to the permeability (K scale), they are:1.Tight : < 5 md2.Fair : 5 – 10 md3.Good : 10 – 100 md4.Very good : 100 – 1000 md

2.4. Acid solutibilityAcidizing is the way to

stimulate the reservoir consisting of limestone or dolomite reservoir rocks and acid dissolvable rock. This is done by injecting acid into the reservoir to get the price of the permeability and porosity of the

larger or more commercial in an oil field.

The acid used is 15% of the acid chloride reacts with carbonate rocks according to the equation:

CaCO3 + 2 HCl CaCl2 + H2O + CO2

Prior to stimulation with acidizing must be planned precisely laboratory data obtained from samples of formation fluid and the fluid reservoir stimulation so that the information obtained from the laboratory can be used to plan operations with proper stimulation. And, in turn, increase the productivity of the formation can be obtained as expected. One of the required information is the solubility of the acid rock samples (acid solubility). This method uses a gravimetric technique for determining the reactivity of the acid formation.

Acidification method (Acidizing) can be divided into two kinds:

1. Matrix Acidizing, in this method, the acidic solution is pumped or injected in order to dissolve the rock and sediment formations around the wellbore. The pressure used is smaller than the fracturing pressure of the formation. Thus, the acid can react with the walls of the pores of the rock and eventually will enlarge pores. The maximum radial penetration and acid solution is dependent on the speed of the acid in the pores. Some of the assumptions used in the implementation of this method are as follows: a. Homogeneous formationb. Uniform pore size

c. Penetration of acid solution uniformly and radially d. Reaction speed decreases uniformly with decreasing acid concentration e. The weight of limestone is dissolved on any additional distance decreases uniformly until all the acid used. Based on the above assumptions, the radial distance of the acid solution will penetrate the formation before the acid solution used up entirely, can be formulated by the following equation:

Injected acid volume = volume of pores invaded :

qi . t = ϕ . h . (ra2 – rw2)...........(9)

ra = htqi

..

+ rw2 …….....(10)If qi is expressed in barrels / min and t in seconds, then the equation can:

ra = htqi

..60.5615

+ rw2…...(11)

where: ra = radial distance of acid penetration, ft. φ = porosity, fraction. qi = acid injection rate, bbl / min. rw = radius of wellbore, ft. h = thickness of the formation.

In the above equation, the unknown factor is spending time, which must be specified in laboratory.spending time depends on the ratio of the rock

with an acid solution, which is called the surface area.For specific matrix acidizing the area are:

S f = 102 x 102 KF..21

..(12)where: K = permeability (darcy) Sf = specific surface (cm2 / cm3) F = factor formation custody

To get the fluid penetration and good acid, necessary reduction reaction rate and raise the rate of injection of acid solution into the formation. Spending time acid depends also on the pressure, temperature, speed and acid in retarding from the additives of the rock.

3. EXPERIMENT PROCEDURE

3.1. Porosity

Porosity measurements by weighing method. Started by weighing the dry cores in a bowl, for example, the dry core weight = W1 grams. The second take a core that has been saturated in kerosene kerosene then weigh them, eg severity = W2 grams. Taking the third core (which is still saturated with kerosene), then weighed in air, such as severity = W3 grams. The latter calculate Vb, Vg, Vp.

3.2. SaturationSaturation measurements by

the method of distillation. First of all take a fresh core or that has been saturated with water and oil. The second considers the core, for example weighs a gram. The third insert the core into the Dean & Stark flask filled with toluene. Then we fill with water

trap and a reflux condenser. The fourth heat for approximately 30 minutes until the water is no longer visible. Cool it and read the fifth volume of water being stored in the water trap, the trap eg cc = b = b g. Drying sixth sample in the oven for 15 minutes, then cool it in exicator. Then weigh the dry cores, eg = c g. The seventh Then we can calculate the weight of the oil to the equation "a - (b + c) g = d g". Next calculate the volume of oil. And lastly calculate oil saturation and water saturation.

3.3. PermeabilityTo measure liquid

permeability, using a liquid permeameter. First insert the core into the core holder. The second filling burette with water (test liquid). The third open valve core holder and burette filled. The fourth shut off valve. The fifth set the desired pressure on the pressure gauge to set the pressure regulator. Next restore the fill valve discharge to discharge. The latter recorded the time required to drain fluid from the upper limit to the lower limit burette, and calculate permeability. To calculate the gas permeability, we use gas permeameter. First of all make sure regulating valve closed, connect the gas line to the gas entering inlet. Both cores on the core holder. The third play flowmeter selector valve on the "large". The fourth open the regulating valve, rotate until the pressure gauge shows the

number of 0.25 atm. The fifth pick in the reader flowmeter range between 20-140 division. Next if under 20, turn the selector valve to the "medium" and raise the pressure to 0.5 atm. If the reader on the flowmeter under 20, turning the selector valve to the "small" and raise the pressure to 1.0 atm. If the flowmeter remains up from number 20, stop the experiment and check cores in core holder. If the flowmeter shows a number above 140 in the "large" cane, the permeability of the core is too large. Experiments we stop or try raising long cores or reducing the cross sectional area of the core. Then record the temperature, pressure and flowmeter readings. Do not forget to change the pressure to 0.25 atm with regulators. The latter experiment repeated 3 times.

3.4. Determination of formation samples dissolved in acid solution

The first thing to do is prepare the carbonate rock samples that have been dried and weighed.The second is to prepare a solution of 5N HCl acid for the carbonate rocks. The third, insert the carbonate rocks into HCl solution for 10 minutes. Next , insert the stone into the oven for ± 20 minutes to dry. And than, weigh the carbonat rock. Calculated the solubility as weight% and the solubility of the material dissolved in 5N HCl

4. DISCUSSIONPorosity measurements or

samples of reservoir rocks is to know the size of the cavity in the rock by rock bulk volume and pore volume of the rock. In the process of exploitation, rock porosity criteria that will be produced is effective porosity

rocks or in other words the ratio of the pore volume that related to the total volume of rock. In this experiment a reservoir rock porosity measurements, carried out in two ways: with mercury injection and by weighing. But we only use the weighing method only, due to damage of mercury injection apparatus. Where in this experiment the number of porosity obtained from experiments with the weighing method obtained at 18.375% effective porosity, thus belonging to the good porosity.

Measurement of Fluid Saturation its purpose to determine the pore space of reservoir rocks containing fluid especially oil. The pores of the reservoir rock containing fluid usually consists of oil, water and gas. This is related to the estimated size of the reserve reservoir. In this experiment, fluid saturation measurements performed by the method of distillation. From the experiment, obtained oil saturation (So) of 0.8824, water saturation (Sw) of 0.1176, and gas saturation (Sg) of 0. This indicates that the core sample saturated by two kinds of fluids, oil and water. Where there is no gas saturation, so that the gas saturation is 0.

Permeability measurements are used to determine the ability of a fluid reservoir rocks passed without damaging the structure of the constituent rocks. With permeability measurements we can determine the fluid flow

rate. In experiments using the absolute permeability permeameter measurements were performed with two methods: liquid permeameter and gas permeameter which each method performed 3 times in order to get the price of permeability K1, K2 and K3. Getting a number of permeability with Liquid permeameter method as follows K = 0.0236 darcy. Whereas the gas permeameter method obtained price = 0.01235 darcy permeability K1, K2 and K3 = 0.01395 = 0.0125 darcy darcy. Where K is the value of K that has not been corrected by the effect Klikenberg. When the amount of permeability is inserted into the table K vs. 1 / P then expressed in the graph. From the graph obtained linear equation y = 0,0004x + 0.0126. The equation used to find the factors Klikenberg permeability (K *). K * generated at 0.1329.

Determination of formation samples dissolved in acid solution, determine the levels of soluble formation in acid solution, in order to obtain important information prior to stimulation. In this experiment the aim to clean up or dissolve impurities reservoir rocks with an acid solution, can also increase the average rate of production. Acidification is done in this experiment using 5 N HCL with concretration due to the nature of HCL that reacted with carbonate. From this experiment price percent solubility obtained was 15.78% or mean pore blockage in the core of 15.7% acid.

5. CONCLUTION Of all the experiments that have

been carried out in general it can be concludand that:

1. Analysis of the core is an attempt to get information about the physical properties of rocks. In its application is very useful for a lot of help in the withdrawal of oil and gas, particularly in deciding how best to do the production.

2. Porosity shows the amount of pores in the rock cavities that can be occupied by the fluid.

The surface obtained is effective porosity. In experiments obtained 18.375% porosity sandstone with the weighing method. Some factors affecting the porosity is: quantity or temperature which is composed of rock, grain uniformity, shape and structure of the rock, cementation, compaction, angularity, packing, fracture due to secondary processes.

3. Definition of fluid saturation, fluid saturation is the ratio of the pore volume occupied by a

particular fluid with the rock pore volume. Saturation is composed of water saturation

(Sw), oil saturation (So), gas saturation (Sg). From the obtained trial So = 0.1176; Sw =

0.8824; Sg = 0.

4. The larger permeability of the reservoir rock, the less time needed to fluid through the pores rock which are interconnected, so that the production rate will be expand. In the experiment get permeability (K) in the gas permeameter tool that is: ΔP =

0.25 atm to K = 0.01235; ΔP = 0.5 atm K = 0.01395 Darcy,; ΔP = 1 atm K = 0.0125 Darcy. In liquid permeameter, ΔP = 1 atm obtained K = 0.0236 Darcy. Permeability is closely related to the porosity, so it can be concluded that permeable rock shaft but not necessarily always the opposite. Several factors influence permeability

5. The solubility degree of the sample in acid solution is 15,78%. So, if we do the acidizing using HCl, most of the depositions will be soluble in acid solution. It is proper to do the acidizing using HCl.storage by reinjection into reservoir formations. Above measuring petrophysics, we can indicates that limestone which we take is bad to be a reservoir rock

6. The rock samples have high porosity values and permeability values are low because of the rock sample is included in the classification of clastic limestone carbonates where porosity is usually obtained intergranuler porosity type, which is sometimes also enlarged by the dissolution. Porosity can reach as high as 18,375 %, but only has K = 0.0236 darcy permeability.

Reference1. Buku Petunjuk Praktikum

Analisa Inti Batuan, Laboratorium Fakultas Teknologi Mineral, Jurusan Teknik Perminyakan, Universitas Pembangunan Nasional “Veteran” Yogyakarta.2013.

2. Tiab , Djebbar and Donaldson Erle C. Petrophysics : theory and practice of measuring

reservoir rock and fluid transport properties. United States of America, 2004.

Appendix

Rocks in the field (Kemadang area) and core samples

Korea National Oil Team in Kemadang area

Kemadang Village Map

Laboratory tools

Porosimeter, analog scale, and digital scale

Dean & Stark aparatus, gas permeameter, liquid permeameter, and oven.

Graphic 1/P vs K