The first step towards refilling your XP15000 is to convert it to being a chipless machine. This requires that you download and install new programming into your printer so that it now disregard sand no longer requires cartridge chips to work.  (Epson uses the chips to prevent refilling!).
One such place to acquire this is the following:
One also needs to purchase an activation key  in order to complete the process:
The process works though the documentation is somewhat incomplete in that it will indicate an update failed at 99% or so. Ignore this and Click on Next and proceed to the next step. It should complete properly at that stage.
If the printer prints blank pages. Unplug the printer for 5 minutes and reattempt. It should eventually be OK.
Please remember I  am not affiliated with the software provider in any way.

REFILLING:  We highly recommend reusing the OEM tanks provided by Epson as they are the best ones suited to the XP15000 by far. All the tanks provided by Epson, Standard Sized, Setup ( provided with the printer) and the largest XL sized can be refilled to the SAME VOLUME/weight. The ideal volume to refill to is approximately 26 to 27 grams.  The standard OEM tanks are 17.5 grams. Setup is 20.3 grams and the XL are 26.3 grams.

To refill the tanks, simply remove from the printer and then invert so the ink pad  faces UP. Now carefully squeeze the ink bottles and direct the ink towards the pad. The cartridge will quickly and immediately absorb the ink back into the cartridge. Continue doing this till until the cartridge weighs 26 to 27 grams. If you stream the ink without a scale to tell you when to slow down and stop, then you will depend on carefully looking at the pad. As soon as it glisten and looks wet, you will immediately stop streaming. Thereafter take a kitchen paper towel and touch the pad and absorb ink till the wet look is gone. Now carefully reinstall the tank.
First is that you should never use the tanks till they are completely empty. Each time you do so, the tanks will reabsorb less ink when refilling. This Is not ideal and slowly the tank will not reabsorb ink as much ink as it is supposed to. This can be rectified with advanced instructions. As soon as you reprogram the printer, you should immediately refill the tanks to 27 grams.
If you purchase new tanks, there is no need to purchase the XL sized tanks. Purchase the standard tanks and immediately refill them to the 27 grams.
Since the chip feature which estimates the remaining ink level is disabled, you will need to manually check the ink level periodically to ensure that you never run out of ink. When you do, should always top off ALL the tanks at the same time. Each time the lid is opened and a tank removed, the printer will consume 2.4ml of ink before it can print again. This happens when you removed ONE or SIX tanks. So to minimize ink waste, as long as you open the printer lid and check ink levels, you should always fill ALL up.
There is a maintenance box at the right hand corner of the printer which provides access to a waste ink tank or maintenance box. This can hold up to 80ml of ink or so, This allows approximately 30 refill processes to occur. Since the cost of this box from Epson is a reasonable $10 USD one should not be scared to check and refill if you are unsure of the ink levels in the tank. If you refill the tanks every month, the total cost of wasted ink and the Epson maintenance box will cost less than $20 over two years!!!!!!
Spend some time watching this video: You will understand the whole process!